Because they are compromising the Bible for the sake of culture.
Let me give you an example. Israel dwelled in a pagan Egypt for over 400 years, and took on much of the culture they lived in, forgetting their God. They were taken into another land where other cultures utterly alien to their own, and to the one God was establishing through the Mosaic covenant with its laws. He did not change, no matter how much they had. And He destroyed those cultures who lived contrary to His holiness and commands.
The Israelites were surrounded by other cultures even when they possessed the land. He warned them to not worship their gods, or commit the abominations they committed. God Himself told them what those abominations were, which included sexual abominations. God did not give them permission to take on those cultural aspects that were against His law. He did not change or compromise for the sake of anything.
The Israelites did not listen and began doing the same things as the culture around them. What did God do? Did He change or did He send judgement? Since the cross, there have been constant cultural changes in the world. The 1950's cultural norms were different than the 40's. the 60's than the 50's and so on and so on. Did God change with any of those changes? Visible churches are not God. so what they do is not how we measure the word of God. Everything built on a different foundation than the one laid by Jesus and the apostles----which is the foundation given by God----will be burned up in the judgement. (1 Cor 3:11-15) The only thing that will remain is Jesus Christ. And that means all that He is, and said, and did, not "Yes Jesus, but only those things I am willing to accept and that do not interfere with my fleshly desires." If that is the way a person looks at it, then Jesus is not their Lord.