There were many prophecies that needed to be fulfilled. Daniel is one as it relates to time but there was also a prophecy about place:
2 And Thou, Bethlehem
Ephrata, art a little one among the thousands of Judah: out of thee shall he come forth unto me that is to be the ruler in Israel: and his going forth is from the beginning, from the days of eternity.
Mary and Joseph were from Nazareth. They went to Bethlehem because of the census that had been ordered to follow. If Jesus had been born a year earlier or later, odds are they wouldn't have been journeying to Bethlehem.
We can get lost in prophetic aspects of this and lose the theological point found in Galatians.
4 But when the time had fully come, God sent his Son, born of a woman, born under law, ..."
He came because it was the right time. God not only worked through Israel, but he worked through the gentiles. Throughout the centuries the Jews had been spread throughout the known world. Paul, went preaching in gentile cities, would preach to the Jews. These Jews had established communities and synagogues and many gentiles had been exposed to their belief in a one true God. They didn't convert, but they were called God-fearing, so these gentiles were prepared or predisposed to hear the gospel. The Roman Empire had built a system of roads and communication, which made it possible to spread the gospel. Prior the Romans, the Greeks had developed a system of philosophy which prepared the way for the profound theological truths of the gospel mysteries to be explained.
So in answer to your question, He came when the stage had been set.