In Catholic exorcisms, why are the Holy Angels (such as the possessed's guardian angel) are not enlisted to assist?
I do not know where you got that idea from, but I assure you the opposite is the truth.
First of all (Fr.) Malachi Martin has a very dubious past. He may have written the book entitled Hostage to the Devil: The Possession and Exorcism of Five Contemporary Americans. However he likes the sensational and was often at odds with the Church. Personally, I do not trust his fact findings and I am not about to dive into this can of beans here.
Does the following sound like a first class Churchman:
Disillusioned by Vatican II, he asked to be released from certain aspects of his Jesuit vows in 1964 and moved to New York City, where he later became an American citizen.
His 17 novels and non-fiction books were frequently critical of the Vatican hierarchy, whom he believed had failed to act on the Third Prophecy revealed by the Virgin Mary at Fátima. Among his most significant works were The Scribal Character of The Dead Sea Scrolls (1958) and Hostage To The Devil (1976) which dealt with Satanism, demonic possession, and exorcism. The Final Conclave (1978) was a warning against Soviet espionage in the Holy See via Soviet spies in the Vatican.
Martin claimed that Popes John XXIII and Paul VI were Freemasons during a certain period and that photographs and other detailed documents proving this were in the possession of the Vatican State Secretariat. He allegorically mentioned these supposed facts in his 1986 novel Vatican: A Novel, where he related the Masonic adherence of Popes Giovanni Angelica and Giovanni De Brescia. He claimed Archbishop Annibale Bugnini C.M. was a Freemason and that Agostino Casaroli, long-time Cardinal Secretary of State, was an atheist.
He spoke and wrote often about the Three Secrets of Fátima and was an ardent supporter of Fr. Nicholas Gruner: "Father Gruner is fulfilling a desperately needed function in the ongoing perception of Mary's role in the salvation of our imperiled world. Father Gruner is absolutely correct that the consecration of Russia as - Our Lady desired, has not been executed". - Malachi Martin (Wikipedia)
Personally I put no stock in Malachi Martin’s work. Many think he even had affairs with married women. He staunchly supported Fr. Nicholas Gruner who was suspended by his own bishop! Fr. Nicholas Nightingale Gruner personally appealed to Rome about the suspension of his priestly duties, but was unsuccessful.
Now let us take up the crux of your question. In Catholic exorcisms, why are Holy angels (such as the possessed's guardian angel) are not enlisted to assist?
Fr. Gabriele Amorth makes note in his books that the praying to the Holy Angels and Guardian Angels in particular for help during exorcisms is beneficial. Both Malachi Martin and Fr. Amorth recommend reciting the rosary during exorcisms.
Please remember that more often than not, official exorcisms take hours to get through. Priests, in these times often employ other prayers not mentioned in the Rituale Romanum.
The Litany of the Saints is prayed as part of the Ritus Exorcizandi Obsessos a Dæmonio. It is a very common practice to add Patron Saints or Saints of special devotion to a location or person at times of priestly ordination or solemn profession of a religious. There no reason prohibiting the inclusion of patron saints or the Guardian Angels to be included in the Litany of the Saints during an exorcism.
More to the point is that Pope Leo XIII’s prayer Exorcismus in Satanam et Angelos Apostaticos is in fact a Prayer to St. Michael the Archangel.
In nómine Patris, et Fílii, et Spíritus Sancti. Amen
Ad S. Michaelem Archangelum Precatio
Princeps gloriosíssime cæléstis milítiæ, sancte Michaël Archángele, defénde nos in
prǽlio advérsus príncipes et potestátes, advérsus mundi rectóres tenebrárum harum, contra spirituália nequítiæ, in cæléstibus. Veni in auxílium hóminum; quos ad imáginem similitúdinis suæ fecit, et a tyránnide diáboli emit prétio magno. Te custódem et patrónem sancta venerátur Ecclésia; tibi trádidit Dóminus ánimas redemptórum in supérna felicitáte locándas. Deprecáre Deum pacis, ut cónterat sátanam sub pédibus nostris, ne ultra valeat cáptivos tenere hómines, et Ecclésiae nocére. Offer nostras preces in conspéctu Altíssimi, ut cito antícipent nos misericórdiæ Dómini, et apprehéndas dracónem, serpéntem antíquum, qui est diábolus et sátanas, et ligátum mittas in abýssum, ut non sedúcat ámplius gentes.
In Nomine Jesu Christi Dei et Dómini nostri, intercédente immaculáta Vírgine Dei
Genetríce María, beáto Michaéle Archángelo, beátis Apóstolis Petro et Paulo et ómnibus Sanctis, et sacra ministérii auctoritáte confísi, ad infestatiónes diabólicæ fraudis repelléndas secúri aggrédimur.
EXSURGAT Deus, et dissipéntur inimíci ejus, et fúgiant qui odérunt eum, a fácie
The act of exorcism within ritual is only part of the equation and takes about 20 minutes to recite, but the act of exorcism itself goes on for much longer. There is lots of time for additional prayers for the exorcist to invoke the Guardian Angels of all involved. This is the practice of Fr. Gabriele Amorth himself!
More than not, many exorcisms are needed to liberate a soul from the Demon. All involved would most naturally pray to Our Lord, His Mother, their Guardian Angels and their patron saints for assistance. The exorcists that I know actually encourage this.
Only the exorcist is permitted to recite the ritual and interrogate the Demon(s) involved the exorcisms. The exorcist must instruct all involved in what to do and not to do. This would also encompass what additional prayers one could silently recite during an exorcism.