Why was the old covenant changed to the new covenant?

First the old covenant was just with the Jews. It provided a system of laws which no one could keep. God knew that no one could keep the laws and that is why in the Bible it declares the coming of Jesus. God then created the new covenant which applies to all who believe in Jesus as Lord and Savior. Remember God does not see time as we do so the new covenant has been in play for all time from the beginning.