Interesting question Crazy Guy especially when one considers that the Paulines (who owed their origin to the Apostle Paul) was the group among the early Christianities that won out and became the dominant force in Christianity YET there was in fact an irreparable gulf between Paul and the Jerusalem Church as Paul was teaching his Gentile converts that with the advent and mission of Jesus, there was no longer a requirement to observe Jewish practices such as their dietary constraints whereas men from Jerusalem were telling Paul's Gentile converts that they had to follow the Laws set down by Moses. These men were teaching that as followers of Jesus, Paul's Gentile converts had to be Jews.
So one wonders why the Torah was not scrapped given Paul's differing view point regarding some Torah issues.
It seems to me however, that the reason why the Torah, and by extension, the entire Old Testament (OT), was retained by Christianity, was because of the need to prove that Jesus Christ was the person who fulfilled the OT prophecies regarding the Messiah.
This is however, not the case, since the prophets of old did not speak of a second coming of the Messiah. That the Messiah would come into the world but then have to come again is a New Testament (NT) idea. The NT writers forced the OT to conform to the NT - this is one of the reasons why the NT can be used as a Jewish polemic against Christianity.