Because the earliest Christians, when they were rejected by the Jews since Christian theology contradicts the Bible, went to the non-Jews to obtain converts.
They quickly discovered that the non-Jews would not convert, unless the requirements of Gd were removed, especially those of observing the Sabbath by not working, and of keeping kosher, and of circumcision.
When they wrote the Christian's New Testament, they wrote verses they attributed to Jesus and to Paul to make observance of the Laws of Gd, like the Sabbath and of keeping Kosher and circumcision, no longer required. Then, the new faith of Christianity became more popular among the pagans of Europe.
Of course one should ask: What would Jesus have thought of this development? After all, Christians believe that Jesus also said,
Matthew 5:17-19 17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. 18 For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. 19 Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
Last i looked, heaven and earth have not passed away, so, to Jesus the Laws of Gd are to be followed!
And if one person perfectly fulfills the laws of stopping at a stop sign, does this meant that from now on, no one has to stop at any stop sign anywhere? Fulfilling the law, even perfectly, does not mean the law is no longer valid or to be followed!
If Jesus said that those who teach others to break the commandments will be least in Heaven, it must mean that Jesus thought one should continue to observe God's laws. Unless being least in heaven is good enough.
Then, as the followers of Christianity became more and more and more from the non-Jewish people, remembering that only the Jews were commanded to keep these laws, anyway, the Laws became irrelevant anyway.