Objection 1. It would seem that the Old Law was not suitably given at the time of Moses. Because the Old Law disposed man for the salvation which was to come through Christ, as stated above (2,3). But man needed this salutary remedy immediately after he had sinned. Therefore the Law should have been given immediately after sin.
Objection 2. Further, the Old Law was given for the sanctification of those from whom Christ was to be born. Now the promise concerning the "seed, which is Christ" (Galatians 3:16) was first made to Abraham, as related in Genesis 12:7. Therefore the Law should have been given at once at the time of Abraham.
Objection 3. Further, as Christ was born of those alone who descended from Noah through Abraham, to whom the promise was made; so was He born of no other of the descendants of Abraham but David, to whom the promise was renewed, according to 2 Samuel 23:1: "The man to whom it was appointed concerning the Christ of the God of Jacob... said." Therefore the Old Law should have been given after David, just as it was given after Abraham.
On the contrary, The Apostle says (Galatians 3:19) that the Law "was set because of transgressions, until the seed should come, to whom He made the promise, being ordained by angels in the hand of a Mediator": ordained, i.e. "given in orderly fashion," as the gloss explains. Therefore it was fitting that the Old Law should be given in this order of time.
I answer that, It was most fitting for the Law to be given at the time of Moses. The reason for this may be taken from two things in respect of which every law is imposed on two kinds of men. Because it is imposed on some men who are hard-hearted and proud, whom the law restrains and tames: and it is imposed on good men, who, through being instructed by the law, are helped to fulfil what they desire to do. Hence it was fitting that the Law should be given at such a time as would be appropriate for the overcoming of man's pride. For man was proud of two things, viz. of knowledge and of power. He was proud of his knowledge, as though his natural reason could suffice him for salvation: and accordingly, in order that his pride might be overcome in this matter, man was left to the guidance of his reason without the help of a written law: and man was able to learn from experience that his reason was deficient, since about the time of Abraham man had fallen headlong into idolatry and the most shameful vices. Wherefore, after those times, it was necessary for a written law to be given as a remedy for human ignorance: because "by the Law is the knowledge of sin" (Romans 3:20). But, after man had been instructed by the Law, his pride was convinced of his weakness, through his being unable to fulfil what he knew. Hence, as the Apostle concludes (Romans 8:3-4), "what the Law could not do in that it was weak through the flesh, God sent [Vulgate: 'sending'] His own Son... that the justification of the Law might be fulfilled in us."
With regard to good men, the Law was given to them as a help; which was most needed by the people, at the time when the natural law began to be obscured on account of the exuberance of sin: for it was fitting that this help should be bestowed on men in an orderly manner, so that they might be led from imperfection to perfection; wherefore it was becoming that the Old Law should be given between the law of nature and the law of grace.
Reply to Objection 1. It was not fitting for the Old Law to be given at once after the sin of the first man: both because man was so confident in his own reason, that he did not acknowledge his need of the Old Law; because as yet the dictate of the natural law was not darkened by habitual sinning.
Reply to Objection 2. A law should not be given save to the people, since it is a general precept, as stated above (90, A2,3); wherefore at the time of Abraham God gave men certain familiar, and, as it were, household precepts: but when Abraham's descendants had multiplied, so as to form a people, and when they had been freed from slavery, it was fitting that they should be given a law; for "slaves are not that part of the people or state to which it is fitting for the law to be directed," as the Philosopher says (Polit. iii, 2,4,5).
Reply to Objection 3. Since the Law had to be given to the people, not only those, of whom Christ was born, received the Law, but the whole people, who were marked with the seal of circumcision, which was the sign of the promise made to Abraham, and in which he believed, according to Romans 4:11: hence even before David, the Law had to be given to that people as soon as they were collected together.
Source: Summa Theologica (New Advent)