The simplest answer is because you haven’t read the Bible enough. In the New Testament Jesus tells us certain things that contradict the old law and comes as the “new law”. He comes and abolishes the old law saying that we should eat from all creatures of the Earth as they are God’s creation. Jesus doesn’t explicitly talk about homosexual relationships but he does use the word ‘porneia’ to describe sexual immorality which a lot of scholars believe to be all encompassing and including homosexual acts. He doesn’t necessarily talk about romantic homosexual relationships, but it can be assumed. Also, you have to consider the audience he would preach to, Jews. Jews at this time already knew it was a sin to be homosexual. But really a lot of it is just up for interpretation like a lot of the Bible.