In the OT God was teaching the Isrealites how to live outside of slavery. He was avoiding what happened to the freed slaves of America just after the Civil War. Being slaves they had no idea of how to live outside of a slaves exisitance. That's why God was so specific with the just freed hebrews. By the time of the NT that situation had changed, and the laws had gotten worse. The pharisees, sadducees and levites had added so many new laws that living was near impossible. It was almost impossible to live in such a way where a law was not broken. That required a new teaching, and thus Jesus' message. Why? Because there were over 1500 (approx. I'm not positive) laws concerning the sabbith alone!! The Isrealites had gone from not knowing howto live free to not knowing how to live by God's law. Sorta the same thing in my thinking. So, almost literally, God was again "freeing" them from slavery. It's just that this time it was selfimposed.