What is the Catholic perspective on why are Angels and Archangels at the bottom of the "Angelic Hierarchy"?
This is one of those questions, I want to say that I know how to answer, but at the same time there are many nuances to be addressed here.
Possibly I see two reasons for this. There are nine choirs of angels: Seraphim, Cherubim, Thrones, Dominations, Virtues, Powers, Principalities, Archangels and Angels. The angelic beings within the nine choirs of angels are angels; but not all angels for example are Seraphs. This is equally true of all angels of the higher ranks of angelic beings others than Angels properly speaking of.
A second reason seems to be in the fact that the higher the rank of angels the greater their intimacy with God and the Divine as well as their responsibilities before the Divine Majesty of the Sacred Trinity. For example Seraphs tend the altar of God and Angels help men.
It is now generally accepted that there are nine choirs of angels and each choir is of angel is different and ranked accordingly.
During the Middle Ages, many schemes were proposed, some drawing on and expanding on Pseudo-Dionysius, others suggesting completely different classifications. According to medieval Christian theologians, the angels are organized into several orders, or "Angelic Choirs”.
Pseudo-Dionysius (On the Celestial Hierarchy) and Thomas Aquinas (Summa Theologica) drew on passages from the New Testament, specifically Ephesians 1:21 and Colossians 1:16, to develop a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three Orders or Choirs. Although both authors drew on the New Testament, the Biblical canon is relatively silent on the subject, and these hierarchies are considered less definitive than biblical material.
Choirs in medieval theology
St. Thomas Aquinas in Summa Theologica (1225–1274):
Seraphim, Cherubim, and Thrones;
Dominations, Virtues, and Powers;
Principalities, Archangels, and Angels.
Christian angelology (Wikipedia)
There is no specific passage in the Bible that lists out the nine orders of angels there are parts of Scripture that speak to an angelic hierarchy.
There may be yet another reason for angels being at the bottom of the Angelic Hierarchy. There deal with the affairs of men, by protecting us against the temptations of the Devil whereas the higher Orders of Angel seem to deal with the affairs of God more intimately.
The seraphim are the angels closest to God. As such, they reflect most immediately the highest attribute of God manifest in creation: His love. They are on fire with the love of God; the very name means “incandescent ones” or “burning ones.” Classical sacred art portrays them as entirely red and ablaze. They are usually depicted as having six wings but no faces — simply a sea or ring of flame around the Holy Trinity. Because of this burning love, more than any other angel they have the most perfect knowledge of God, which makes them the most perfect adorers. St. Jerome notes that they not only burn by themselves, but they also inflame others with the love of God.
According to the prophet Isaiah, the seraphim are the angels whom he hears crying out “Holy, holy, holy,” as one of them purifies Isaiah’s mouth with a coal from the altar so that he might serve as the Lord’s messenger (Isa. 6:3–8). In the Extraordinary Form of the Roman Mass, the priest evokes this moment as he prays for worthiness in proclaiming the Gospel. We too should pray to the seraphim that we might be purified in our responsibilities as teachers and bearers of the Word to our families, our friends, and all those over whom we have responsibility. It was a seraph who appeared to St. Francis of Assisi when he received the stigmata. Later mystics, too, will speak of the seraphim as the Lord’s messengers and intermediaries when they had extraordinary experiences of loving and transforming divine union.
The cherubim have a deep intellectual knowledge of divine secrets and of the ultimate causes of things; their name means “all-knowing one.” As such, they constantly contemplate the wisdom and the love of God in His relationship with mankind. They reflect His omniscience. The cherubim were the mighty adorers of the first covenant in its wisdom; images of the cherubim were the only images of beings that were permitted in the ancient Temple of Jerusalem. Their carved figures adorned the lid of the Ark of the Covenant, which prefigured both the Virgin Mary “tabernacling” the unborn Christ and the Eucharistic tabernacles of our churches, containing the new manna of Christ’s sacramental Body and Blood. Embroideries of the cherubim also covered the beautiful drapery that separated the Holy of Holies from the outer court of the Temple. It was that veil that was ripped from top to bottom when Our Lord died on the Cross as the sign that He had passed into the Eternal Sanctuary and that the Temple of Jerusalem had fulfilled its purpose (Matt. 27:51). The cherubim are still considered protectors of the New Covenant and so are often depicted on tabernacles and Eucharistic vessels.
The thrones, as their name suggests, can be thought of as beings raised up to form the seat of God’s authority and mercy. A throne manifests the glory and authority of a king; it expresses stability and power. And since a throne is also a judgment seat, these angels are especially concerned with divine judgments and ordinances.
In the early Church, a common representation of God’s glory in Heaven was a mosaic behind the altar and above the seat of the bishop that represented an empty throne with a radiant cross mounted above it. This image represented Christ the King, Lord of all and Judge of the living and the dead. But His judgment seat was also a throne of mercy, for Christ has redeemed the world by His Cross. His love has brought us to salvation. The thrones are never seen or experienced as “flying” but as “rolling” across the heavens, in keeping with their manifesting the Lord’s stability.
The dominations are concerned with the government of the universe. They are the first of the three choirs in the second ring, which is the ring of the cosmos — the angels who are charged with great and universal stewardships. The dominations in particular are involved in the workings of divine power. They coordinate the ministries of all the angels who deal with creation. We see in the angelic world that the Church’s teaching that God works through secondary causes is beautifully demonstrated. The angels mediate God’s power just as the saints intercede for us with Him.
St. Peter mentions the virtues in his first epistle (3:22), as does St. Paul in his Letter to the Colossians (1:16). The name is in some way a mistranslation or at least a “false cognate,” since this choir of angels does not deal with acquired habits (virtues), but rather exercises innate, raw power over the physical universe. According to Pseudo-Dionysius, their name refers to “a certain powerful and unshakable virility welling forth into all their Godlike energies, . . . mounting upwards in fullness of power to an assimilation with God; never falling away from the divine life through its own weakness, but ascending unwaveringly to the super-essential Virtue which is the Source of virtue.”1 They are the lords of causality and the principles of cosmic order in the material realm. They ensure the well-being of the world.
The powers (dunameis) form the third and last choir of the second angelic hierarchy, according to Pseudo-Dionysius, while other scholars and spiritual writers consider them to be the fifth choir. This choir is mentioned occasionally in the Old Testament, such as in the book of Daniel where we read, “Bless the Lord, all powers, sing praise to him and highly exalt him for ever” (Dan. 3:39). Some scholars maintain that the name “powers” is also used to indicate angels in general, since it is the Septuagint’s translation of the Hebrew sabaoth. In the New Testament St. Paul writes that there are powers who have remained faithful to God and powers who have fallen away and become part of the empire of Satan (Eph. 6:12). The choir of powers is thought to introduce man to the higher mysteries while repressing the attacks of the “hostile powers” of Hell against the deepest laws of physical creation.
Princes or Principalities
The princes are also described as having members who have fallen away and others who have remained faithful. Principalities are the leading choir of the last hierarchy of angels. Their activities are described by Pseudo-Dionysius in this way, “The name of the Celestial Principalities signifies their Godlike princeliness and authoritativeness in an Order which is holy and most fitting to the princely Powers.” They are often seen as being the guardians of nations or peoples; this is why St. Michael is described in the book of Daniel as “the prince of Israel,” who comes to the aid of Gabriel against the demonic prince of Persia. It seems fitting that this first choir in the “ring of salvation” should also look after the spiritual structure of the supernatural life of the Church.
This choir is the most known and loved in popular devotion. Among the archangels we find St. Michael, St. Gabriel, and St. Raphael. It is traditionally believed, due to the statements of Raphael in the book of Tobit, that there are only seven archangels.
Three of their names occur in Scripture, and so the Church uses these names in our worship — St. Michael, the prince of the heavenly host and the only one called “archangel” in the Scriptures; St. Gabriel, the messenger of the Incarnation; and St. Raphael, the angel of healing and of medicine.
The names of the other four are not used in our Liturgy, though there are certain churches that preserve these names and make use of them in private devotion, including some Eastern Catholic Churches. Roman Catholics often refer to them as the seven archangels or the seven assisting spirits around the throne of God.
The seven archangels have been regarded from the very beginning as having a special place in God’s plan; their number is often associated with the seven days of the week and the seven sacraments. It is thought that the archangels were outstanding in their fidelity to God, and so in the writings of the saints they are often called archangel princes, an appellation that connotes leadership and authority in the heavenly realm. Many spiritual authors and mystics speak of their special assistance and often attribute other “groups of seven” to their protection or patronage — virtues, gifts of the Holy Spirit, and so on. The archangels are also associated with the protection of nations, dioceses, religious communities, and the mission of the Church.
The ninth and final choir of angels is composed of those who are most involved with the doings of mankind. These angels are those who are sent out on missions from God and from whom the guardian angels are chosen. The angels who fill up this choir may be the lowest, but they are beloved because the Lord places them at our sides to watch over us and to care for us. They are the ministers of Christ’s love and our protectors. They defend us against harm and temptation. They warn us of impending evil and inspire us to remain faithful to God in prayer.
What Are the Nine Choirs of Angels?
St. Thomas Aquinas points out that each angel fulfills his own proper office, but as the angels are far above our natural human knowledge, we cannot know of the distinctions among the angels by our own powers. Thus, because our knowledge of the angels is imperfect, we can only distinguish among them in a general way – and hence, we recognize the angels according to the nine choirs.
Although Fr. John Horgan of my own archdiocese, places St. Micheal, St. Gabriel and St. Raphael as being Archangels and of being from that particular Order of Angels, many Catholics would disagree, and I am one.
The Archangels as a Angelic Rank are above that of the Angels. Thus carry the arch designation in their title. Just as some priests carry the title of archpriest and some deacons have historically been called archdeacons. Many Catholic bishops are of the rank of archbishop.
The ecclesiastical title of archpriest or archpresbyter belongs to certain priests with supervisory duties over a number of parishes. The term is most often used in Eastern Orthodoxy and the Eastern Catholic Churches and may be somewhat analogous to a monsignor, vicar forane or dean in the Latin Church, but in the Eastern churches an archpriest wears an additional vestment and, typically, a pectoral cross, and becomes an archpriest via a liturgical ceremony.
In ancient times, the archdeacon was the head of the deacons of a diocese, as is still the case in the Eastern Orthodox Church, while the archpriest was the chief of the presbyerate of the diocese, i.e. of the priests as a body. The latter's duties included deputising for the bishop in spiritual matters when necessary.
As I mentioned in the beginning of this response to your inquiry, there are many nuances here to be considered, especially regarding St. Michael the Archangel. He is an Archangel, not because he is an archangel, but because he is the Prince of the Heavenly Hosts!
Regarding his rank in the celestial hierarchy opinions vary; St. Basil (Hom. de angelis) and other Greek Fathers, also Salmeron, Bellarmine, etc., place St. Michael over all the angels; they say he is called "archangel" because he is the prince of the other angels; others (cf. P. Bonaventura, op. cit.) believe that he is the prince of the seraphim, the first of the nine angelic orders. But, according to St. Thomas (Summa Ia.113.3) he is the prince of the last and lowest choir, the angels. The Roman Liturgy seems to follow the Greek Fathers; it calls him "Princeps militiae coelestis quem honorificant angelorum cives". The hymn of the Mozarabic Breviary places St. Michael even above the Twenty-four Elders. The Greek Liturgy styles him Archistrategos, "highest general" (cf. Menaea, 8 Nov. and 6 Sept.). - St. Michael the Archangel
The following passages from St. Gregory the Great (Hom. 34, In Evang.) will give us a clear idea of the view of the Church's doctors on the point:
We know on the authority of Scripture that there are nine orders of angels, viz., Angels, Archangels, Virtues, Powers, Principalities, Dominations, Throne, Cherubim and Seraphim. That there are Angels and Archangels nearly every page of the Bible tell us, and the books of the Prophets talk of Cherubim and Seraphim. St. Paul, too, writing to the Ephesians enumerates four orders when he says: 'above all Principality, and Power, and Virtue, and Domination'; and again, writing to the Colossians he says: 'whether Thrones, or Dominations, or Principalities, or Powers'. If we now join these two lists together we have five Orders, and adding Angels and Archangels, Cherubim and Seraphim, we find nine Orders of Angels.
The now famous problem, "The Angel of the Lord," that has engaged the attention of Scripture scholars for decades, may perhaps be solved by admitting that this mysterious Angel of the Lord, is none other than the three Archangels mentioned by name in Sacred Scriptures,Saint Michael, Saint Gabriel and Saint Raphael, God's own legates to His people. The Angel of the Lord appears 65 times in Sacred Scriptures and most certainly does not always refer the same angelic being. The New Testament uses the term "angel of the Lord” (ἄγγελος Κυρίου) several times, in one instance (Luke 1:11–19) identifying it with Gabriel. The mystery goes on!
One could go one, but I think this will suffice to get my point across.