The simple reason is that most of the sweeping changes made by the Church in the early-ish days were to make their religion more mass-marketable as a tradeoff of giving up tenets of the faith in exchange for either getting the key element of salvation out or expanding their power base, depending on who you ask. In a thriving religious free market, Judaism's combination of monotheism with restrictive laws on hygiene and diet would not sell. This was fine for the Jews, who were not particularly evangelistic and had a well-entrenched base of dedicated practitioners, but for a startup cult it's a no-go. Once they'd changed the formula enough, the cult exploded in popularity, and obviously it was the version that chose to overlook or overwrite Jewish tradition that did so. A competing version simply would not be able to survive against it.
I don't think it's that simple. In the 'early-ish' days as you put it, to become a Christian was to risk persecution and martyrdom. Christians were pursued and killed by Romans and also by Temple Authorities. Yet there were over 10,000 Christians at the end of the 1st Century. I don't know how you make that 'mass-marketable'. As for giving up the tenets, I think that would be a very difficult choice as well. Judaic authority in those days was the social fabric - to not practice as a Jew was pretty much the end of your family life and your community life. That would not be easy. On the other hand, our Communion is evolved from the Passover
'Seder' and Baptism almost certainly stems from the Jewish
'mikveh' and
'tevillah'. Jesus himself was baptised prior to the beginning of His ministry; it was not a Christian invention. Both Baptism and Communion were practiced by the early church in the first century.
We also should remember that the apostles, including the skeptic Thomas, and James the brother of Jesus (also a skeptic who later was a leader of the early church in Jerusalem) were all Jewish. (In fact, when Paul came to Jerusalem to donate raised funds for needy Christians there, James insists that he ritually cleanse himself Paul ritually cleanse himself in Herod's Temple to prove his faith and deny rumors of teaching rebellion against the Torah)These men gave up everything including their lives to spread the gospel and start the early church. It is the Apostles firsthand writings that make up much of the New Testament along with the writings of some others who came a little bit later. Christianity was at its root a movement within Judaism; probably not intended to supplant Judaism but something akin to the Reformation. However, as the practice spread among Jews and non-Jews, and as the practitioners distance from the temples and Jewish authorities (necessary due to persecution) continued and increased, the practices evolved. Also, in my opinion, the early Christians were conscious of Jesus' attitude toward temple practices of the time as well as the well-known cooperation between the Temple Authorities and Rome - these formed a pretty solid wedge. Even the skeptics became followers after encountering the risen Christ.
Notable also is the conversion of a great persecutor and in fact, killer of Christians. Saul of Tarsus became the Apostle Paul; the only Apostle not present during Jesus' life. He gave up a title, prestige, a good job and became a HUGE target for the authorities because he was well known. What kind of marketing would that take? Picture Donald Trump giving up his empire, everything he owns, to work for $10 an hour at a non-profit that did outreach to addicts. How would you talk him into it? That's the magnitude of conversion we're kind of talking about. I don't think 'Mass Marketing' or 'Easier Lifestyle' covers this, or any of the other conversions in the early church.
The situation changed once Christianity monopolized the Western market, but by then the damage was done: much like rabid so-called Harry Potter fans who write epic tales about the character who was mentioned once on page 246 and start shrieking like banshees when the books contradict their fanfiction, Christians viewed the Torah as an amusing prequel at best compared to the layer of fan-cruft piled onto it.
This reads like Anti-Christian bigotry to me, and I think you're better than that. Certainly I value both my Jewish friends and the Torah and of course the Old Testament of the Christian bible is about the early history of the Jewish people. So I don't quite understand where that second quote comes from.