Objection 1. It would seem that Christ should not have been born in Bethlehem. For it is written (Isaiah 2:3): "The law shall come forth from Sion, and the Word of the Lord from Jerusalem." But Christ is truly the Word of God. Therefore He should have come into the world at Jerusalem.
Objection 2. Further, it is said (Matthew 2:23) that it is written of Christ that "He shall be called a Nazarene"; which is taken from Isaiah 11:1: "A flower shall rise up out of his root"; for "Nazareth" is interpreted "a flower." But a man is named especially from the place of his birth. Therefore it seems that He should have been born in Nazareth, where also He was conceived and brought up.
Objection 3. Further, for this was our Lord born into the world, that He might make known the true faith. according to John 18:37: "For this was I born, and for this came I into the world; that I should give testimony to the truth." But this would have been easier if He had been born in the city of Rome, which at that time ruled the world; whence Paul, writing to the Romans (1:8) says: "Your faith is spoken of in the whole world." Therefore it seems that He should not have been born in Bethlehem.
On the contrary, It is written (Micah 5:2): "And thou, Bethlehem, Ephrata... out of thee shall He come forth unto Me, that is to be the ruler in Israel."
I answer that, Christ willed to be born in Bethlehem for two reasons. First, because "He was made... of the seed of David according to the flesh," as it is written (Romans 1:3); to whom also was a special promise made concerning Christ; according to 2 Samuel 23:1: "The man to whom it was appointed concerning the Christ of the God of Jacob... said." Therefore He willed to be born at Bethlehem, where David was born, in order that by the very birthplace the promise made to David might be shown to be fulfilled. The Evangelist points this out by saying: "Because He was of the house and of the family of David." Secondly, because, as Gregory says (Hom. viii in Evang.): "Bethlehem is interpreted 'the house of bread.' It is Christ Himself who said, 'I am the living Bread which came down from heaven.'"
Reply to Objection 1. As David was born in Bethlehem, so also did he choose Jerusalem to set up his throne there, and to build there the Temple of God, so that Jerusalem was at the same time a royal and a priestly city. Now, Christ's priesthood and kingdom were "consummated" principally in His Passion. Therefore it was becoming that He should choose Bethlehem for His Birthplace and Jerusalem for the scene of His Passion.
At the same time, too, He put to silence the vain boasting of men who take pride in being born in great cities, where also they desire especially to receive honor. Christ, on the contrary, willed to be born in a mean city, and to suffer reproach in a great city.
Reply to Objection 2. Christ wished "to flower" by His holy life, not in His carnal birth. Therefore He wished to be fostered and brought up at Nazareth. But He wished to be born at Bethlehem away from home; because, as Gregory says (Hom. viii in Evang.), through the human nature which He had taken, He was born, as it were, in a foreign place--foreign not to His power, but to His Nature. And, again, as Bede says on Luke 2:7: "In order that He who found no room at the inn might prepare many mansions for us in His Father's house."
Reply to Objection 3. According to a sermon in the Council of Ephesus [P. iii, cap. ix]: "If He had chosen the great city of Rome, the change in the world would be ascribed to the influence of her citizens. If He had been the son of the Emperor, His benefits would have been attributed to the latter's power. But that we might acknowledge the work of God in the transformation of the whole earth, He chose a poor mother and a birthplace poorer still."
"But the weak things of the world hath God chosen, that He may confound the strong" (1 Corinthians 1:27). And therefore, in order the more to show His power, He set up the head of His Church in Rome itself, which was the head of the world, in sign of His complete victory, in order that from that city the faith might spread throughout the world; according to Isaiah 26:5-6: "The high city He shall lay low... the feet of the poor," i.e. of Christ, "shall tread it down; the steps of the needy," i.e. of the apostles Peter and Paul.
Source: Summa Theologica (New Advent)