In light of scripture, why do Christians disregard the Old…

In light of scripture, why do Christians disregard the Old Testament laws?

“Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them.
For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished.”
Matthew 5: 17 18)

Revelation 21. Starts off like this:
“Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and there was no longer any sea. I saw the Holy City, the new Jerusalem, coming down out of heaven from God, prepared as a bride beautifully dressed for her husband.”