Islam is regarded as a message that completed Judaism and Christianity. That is why it comes last. It is the final update.
All of the prophets who were sent by God had the same message, since the source was one.
Islam is a continuation of Judaism and Christianity because it builds upon what was already taught by the previous prophets. The Previous prophets told their people that there will be a prophet who will come at the end of times and he will complete the faith, referring to RasulAllah. This is why several tribes of Jews and Christians settled Arabia to anticipate the new prophet. When he came many accepted him and many rejected out of arrogance.