Objection 1. It would seem that the Old Law should not have induced men to the observance of its precepts, by means of temporal promises and threats. For the purpose of the Divine law is to subject man to God by fear and love: hence it is written (Deuteronomy 10:12): "And now, Israel, what doth the Lord thy God require of thee, but that thou fear the Lord thy God, and walk in His ways, and love Him?" But the desire for temporal goods leads man away from God: for Augustine says (Qq. lxxxiii, qu. 36), that "covetousness is the bane of charity." Therefore temporal promises and threats seem to be contrary to the intention of a lawgiver: and this makes a law worthy of rejection, as the Philosopher declares (Polit. ii, 6).
Objection 2. Further, the Divine law is more excellent than human law. Now, in sciences, we notice that the loftier the science, the higher the means of persuasion that it employs. Therefore, since human law employs temporal threats and promises, as means of persuading man, the Divine law should have used, not these, but more lofty means.
Objection 3. Further, the reward of righteousness and the punishment of guilt cannot be that which befalls equally the good and the wicked. But as stated in Ecclesiastes 9:2, "all" temporal "things equally happen to the just and to the wicked, to the good and the evil, to the clean and to the unclean, to him that offereth victims, and to him that despiseth sacrifices." Therefore temporal goods or evils are not suitably set forth as punishments or rewards of the commandments of the Divine law.
On the contrary, It is written (Isaiah 1:19-20): "If you be willing, and will hearken to Me, you shall eat the good things of the land. But if you will not, and will provoke Me to wrath: the sword shall devour you."
I answer that, As in speculative sciences men are persuaded to assent to the conclusions by means of syllogistic arguments, so too in every law, men are persuaded to observe its precepts by means of punishments and rewards. Now it is to be observed that, in speculative sciences, the means of persuasion are adapted to the conditions of the pupil: wherefore the process of argument in sciences should be ordered becomingly, so that the instruction is based on principles more generally known. And thus also he who would persuade a man to the observance of any precepts, needs to move him at first by things for which he has an affection; just as children are induced to do something, by means of little childish gifts. Now it has been said above (98, A1,2,3) that the Old Law disposed men to (the coming of) Christ, as the imperfect in comparison disposes to the perfect, wherefore it was given to a people as yet imperfect in comparison to the perfection which was to result from Christ's coming: and for this reason, that people is compared to a child that is still under a pedagogue (Galatians 3:24). But the perfection of man consists in his despising temporal things and cleaving to things spiritual, as is clear from the words of the Apostle (Philippians 3:13-15): "Forgetting the things that are behind, I stretch [Vulgate: 'and stretching'] forth myself to those that are before... Let us therefore, as many as are perfect, be thus minded." Those who are yet imperfect desire temporal goods, albeit in subordination to God: whereas the perverse place their end in temporalities. It was therefore fitting that the Old Law should conduct men to God by means of temporal goods for which the imperfect have an affection.
Reply to Objection 1. Covetousness whereby man places his end in temporalities, is the bane of charity. But the attainment of temporal goods which man desires in subordination to God is a road leading the imperfect to the love of God, according to Psalm 48:19: "He will praise Thee, when Thou shalt do well to him."
Reply to Objection 2. Human law persuades men by means of temporal rewards or punishments to be inflicted by men: whereas the Divine law persuades men by meas of rewards or punishments to be received from God. In this respect it employs higher means.
Reply to Objection 3. As any one can see, who reads carefully the story of the Old Testament, the common weal of the people prospered under the Law as long as they obeyed it; and as soon as they departed from the precepts of the Law they were overtaken by many calamities. But certain individuals, although they observed the justice of the Law, met with misfortunes--either because they had already become spiritual (so that misfortune might withdraw them all the more from attachment to temporal things, and that their virtue might be tried)--or because, while outwardly fulfilling the works of the Law, their heart was altogether fixed on temporal goods, and far removed from God, according to Isaiah 29:13 (Matthew 15:8): "This people honoreth Me with their lips; but their hearts is far from Me."
Source: Summa Theologica (New Advent)