Many say that the NT overrides the OT teachings. That would be like saying that what God said in the OT is no longer truth. That's not possible; for this would make God a liar. He clearly states in Numbers that He is not a man that He should lie.

If eating certain foods was bad for humans in the OT; they remain so in contemporary times. Giving a tenth of your earnings back to God-causes in the form of the tithe, will yield the same promise that is offered by God in OT Malachi.

If God said it is good to rest on the 7th day; and, even demonstrated this as an example for humans, I assure you, it still applies today.

In the NT Jesus reduces the Law Moses taught to two:

Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets. **(Matthew 22:37-40)_

Lastly, the answer to your question of why is because beloved, we live in a fallen world. God has gifted His children with the Spirit of Truth. Too often organized religions become more about what will fill the coffers, than speaking the truth according to God; thus, why some teachings are more popular than others.

That said, we're reminded that He holds teachers of the Word in higher accountability; so, what's misstated by some will hold great consequence on the Day of Judgment.

I hope this helps.

Best regards.

Peace.