What Jesus spoke is the word of God. Everything in the OT and writings in the NT by authors other than Jesus are not necessarily the word of God. They are the words of men. Jesus came to replace what had come before him with a 'new covenant'. He did not come to amend, revise, edit or otherwise modify what had come before. He came to replace it.
Why would we prefer to follow the teachings of Moses and others when we have the teachings of Jesus? Why follow those who followed Jesus teaching in his name other than what Jesus himself taught? Paul? John (Revelations). Jesus said to teach only what he had said and what he had done. He warned us to not believe anyone who taught anything different from what he himself had taught. So how did we end up with several hundreds of 'Christian' denominations divided not by the teachings of Jesus but by the teachings of camp followers preying on Jesus' name and reputation to spread their own ideologies?
The Hebrews were not even monotheists, believers in one god. They believed in many gods.