If God is all-knowing, and is all powerful, what are some theories as to why He didn't send Jesus on Day One?
The short answer is that God willed it so!
Sure God could have chosen a million and one other solutions to this question, but he did not.
For example: Would Jesus Have Come If Adam Had Not Sinned? Why Did He Wait So Long Before Coming?
The following is a Catholic response to this very question according to St. Thomas Aquinas:
Regarding the question of whether Christ would have come if Adam had not sinned, St. Thomas Aquinas (in his Summa Theologica) first states that there are different opinions on the matter. He also notes that God’s power is not limited and therefore God could have become incarnate even if sin had not existed. However, St. Thomas believes that if man had not sinned then the Son would not have become incarnate. As I often do, I’ve presented St. Thomas’ words in bold italics, while my commentary appears in red.
For such things as spring from God’s will, and beyond the creature’s due, can be made known to us only through being revealed in the Sacred Scripture, in which the Divine Will is made known to us. Hence, since everywhere in the Sacred Scripture the sin of the first man is assigned as the reason of Incarnation, it is more in accordance with this to say that the work of Incarnation was ordained by God as a remedy for sin; so that, had sin not existed, Incarnation would not have been (Summa Theologica, Part III, Question 1, Article 1).
While theological speculation may have its place, it is most certain that the Incarnation was instituted by God first and foremost as a remedy for sin. And while the Incarnation offers more than is required to remedy sin (e.g., an increase in human dignity (since God joined our family), God’s visitation, the opening of a heavenly (not merely earthly) paradise), Scripture presents remedy for sin as God’s primary motive. In remedying our sin, God shows the greatness of His mercy, because He does not merely restore us but elevates us to a higher place than before. The least born in to the Kingdom of God is greater that the exemplar of the Old Covenant, John the Baptist. Had we not sinned and had God merely wanted to elevate us, He could have done so in other ways. Hence, St. Thomas’ position is best suited to the evidence.
If the Incarnation is a remedy for sin, why did God wait so long to apply it? St. Thomas provides an answer that is sensible and addresses aspects of the question we might not have considered. His answer is found in the Summa Theologica (part III, question 1, article 5). First he addresses why the Incarnation did not happen before sin:
Since the work of Incarnation is principally ordained to the restoration of the human race by blotting out sin, it is manifest that it was not fitting for God to become incarnate at the beginning of the human race before sin. For medicine is given only to the sick. Hence our Lord Himself says (Matthew 9:12-13): “They that are in health need not a physician, but they that are ill … For I am not come to call the just, but sinners.”
Next, St. Thomas addresses why the Incarnation did not happen quickly, soon after Original Sin, rather than thousands of years later. He sets forth four reasons:
I. Nor was it fitting that God should become incarnate immediately after sin. First, on account of the manner of man’s sin, which had come of pride; hence man was to be liberated in such a manner that he might be humbled, and see how he stood in need of a deliverer. … For first of all God left man under the natural law, with the freedom of his will, in order that he might know his natural strength; and when he failed in it, he received the law; whereupon, by the fault, not of the law, but of his nature, the disease gained strength; so that having recognized his infirmity he might cry out for a physician, and beseech the aid of grace.
Quick solutions to problems do not always permit proper healing to take place. Most parents know that if they solve every problem a child has, important lessons may be lost. It is often beneficial to live with our questions for a while so that the answers are more appreciated and more effective.
Indeed, it took us humans quite a while to really acknowledge the seriousness of our sin and pride. Shortly after Eden, the tower of Babel indicated that human pride was still a grave problem. Even when given the Law, a good thing, the flesh corrupted it, turning perfunctory observance of it into an occasion for pride. The prophets then had to keep summoning Israel and Judah back to the Lord and away from prideful self-reliance. The Assyrian invasion of the Northern Kingdom and the Babylonian Captivity only further illustrated the depths of our sin, so that this cry went up: “O Lord, that you would rend the heavens and come down” (Is 64:1).
We had to be led gradually to recognize our profound need for a savior. Otherwise, even if the remedy were offered, too few might reach for it.
II. Secondly, on account of the order of furtherance in good, whereby we proceed from imperfection to perfection. Hence the Apostle says (1 Corinthians 15:46-47): “Yet that was not first which is spiritual, but that which is natural; afterwards that which is spiritual … The first man was of the earth, earthy; the second man from heaven, heavenly.”
There is a kind of theology of grace implicit in this answer. Grace builds on our nature. And it is our nature, physically and spiritually, to grow gradually. While sudden conversions and growth spurts have their place, the best and most typical growth is that which occurs steadily and in stages.
Thirdly, on account of the dignity of the incarnate Word, for on the words (Galatians 4:4), “But when the fullness of the time was come,” a gloss says: “The greater the judge who was coming, the more numerous was the band of heralds who ought to have preceded him.”
Here is underscored the dignity of the Son of God, that many should precede Him, announcing Him. But there was also a need for us to be prepared to meet Him, so that we would not miss Him or refuse Him when He came. As Malachi says, See, I will send the prophet Elijah to you before that great and dreadful day of the Lord comes. He will turn the hearts of the parents to their children, and the hearts of the children to their parents; or else I will come and strike the land with total destruction (Mal 4:5-6). Those who were prepared were able to abide the day of the Lord’s coming and heed His call.
Fourthly, lest the fervor of faith should cool by the length of time, for the charity of many will grow cold at the end of the world. Hence (Luke 18:8) it is written: “But yet the Son of Man, when He cometh, shall He find think you, faith on earth?”
This is an interesting aspect of the question that many might not consider; we typically ponder more what is good for us than what is good for succeeding generations. But it is sadly true that fervor, both collective and individual, can fade as a wait becomes lengthy. And thus, St. Thomas suggests that God appointed a time for the Incarnation within human history such that the greatest possible number of people could be saved.
Then again Blessed Duns Scotus has his own perspective thinking on this philosophical question:
Would Jesus have become human if man had not fallen?
Most Catholics think the Incarnation is something that happened because of the sin of Adam: God became man to save man from sin. They will often quote the Exsultet to support this position: "O truly necessary sin of Adam, destroyed completely by the death of Christ! O happy fault, which merited for us so glorious a Redeemer!"
This position, however, is not held by all the saints. In fact, this is a point of theological dispute among the Scholastics. One such scholar, Bd. John Duns Scotus, a 13th-century Franciscan, argued that Christ would indeed have become incarnate, even if man had not fallen.
His argument can be summarized in the following syllogism:
If man had not fallen, Christ would not have become Man.
If Christ had not become Man, there would not be any bridge between God and creation. God is no longer the "perfect Man" uniting creation to Himself.
This means that what would have been the highest good of creation (i.e., Christ's human nature) would no longer exist.
Therefore, what is in fact the highest good of creation, Our Blessed Lord's human nature, is the result of an accident, an "occasion of a lesser good," as Scotus says.
But the wise man does not leave the greatest good to chance; on the contrary, it is first in His intention.
But if a wise man intends the greatest good, then a fortiori God, Who is Wisdom Itself, intends the greatest good of creation.
Thus the hypostatic union could not be a result of an accident, and hence its cause cannot be the fall of man, which is clearly not necessary (for else God would directly intend evil, which is absurd). Therefore, God intended to assume human nature and become Man, regardless of whether man fell.
In short, Scotus is saying that God would not leave the greatest work of His creation to chance. For though the Second Person of the Most Holy Trinity is not created, the human nature of Christ is created (albeit in some mysterious, unknowable way).
The point is this: God has predestined certain men to eternal glory (Heaven), and this includes Our Blessed Lord, who is truly Man. In fact, the predestination of Our Lord is prior to every other saint since He is "before all else that is" and "in all things He has primacy" (Colossians 1:17–18).
But the predestination of the saints to glory is not dependent on the Fall of man (it's not as if man needed to fall in order for the saints to attain Heaven). Therefore, Scotus argues, if their glory is not dependent on the Fall, then much less is Christ's glory dependent on the Fall. Therefore, Christ would have become Man had man not fallen.
What about the Exsultet, the happy fault of Adam?
The words of the Easter Vigil hymn do not say Adam's sin was necessary to make God Man, but rather to merit us a Redeemer. If we take this for exactly what it says, then there need not be any contradiction. For had Adam not sinned, we certainly would not have needed a Redeemer. But because Adam did sin, we now have a most glorious Redeemer, Who triumphed over sin and death and crushed the skull of the serpent at the place of the skull.
Sure God could have had many more possible philosophical solutions to this question, but being omnipresent at all times, he certainly knew the best solution for mankind.