Because the Christians believe that Jesus was the summation and conclusive fulfillment of the Law and the Prophets. He was the one prophesied of old who would redeem mankind and save the world, and his teachings thus stood on their own as the core commandment of Jewish Law.
It's NOT supposed to. What we see and hear in the pulpits today is NOTHING like the theology we read in the New Testament!!! JESUS, Paul, Peter, etc. ALL taught in their speeches and writings the things that are in the Old Testament!!!
Because of the fall of the temple. The earliest Christians were EBIONIM (Hebrew word for "the poor") and met in the temple precincts under the auspices of James and Peter. The apostle Paul refers to them as "the pillars". They commissioned Paul to convert and shake down Gentiles for funds for the Jerusalem poor. They seem to have practiced a more Judaic form of Christianity. They have Paul undergo a purification rite. They also waffle a bit on circumcision and kosher practice. But after the fall of the temple, Jews were dispersed into the greater Roman Empire as refugees and slaves. Messianic hopes mushroomed (as they often do in turbulent times) and Messiahs began coming out of the woodwork. Jesus was only one of many candidates for Messiah. But his humble birth and crucifixion made him a "people's choice" candidate--some would say a "populist". Then, he was inflated into the heroic, hallowed, alpha-male Son of God of the Greeks, of whom Apollo, Hercules, and Dionysus are examples. Advancing Christology (Jesus' perceived nature) expanded from that dusty, rural, quixotic, quick-witted rabbi to the divine, world-saving, preexistent, in-dwelling Son of God. And Paul's hyperbolic hermeneutic played a major role in that apotheosis or rise to Godhood. The Gospel of John pretty much clinched it by making Jesus God Himself. The Hellenic, hero-worshiping world demanded a celestial paragon on a pedestal.