Because they are not accepting.
It's quite simple. They tend to be authoritarian, religious, and not accepting of differences. They fear what is different, and worry that it will supplant their way of life. And, you know what? Maybe it could. Who can say?
Personally, I try to walk a middle line. I don't care what you do, as long as A) you're not hurting anyone who doesn't want hurting, and B) you keep it to yourself. And don't ******* try to guilt-trip me. That's probably what helped Donald Trump the most. All the liberal attempts to shame people into submission. It doesn't work. It just doesn't. So stop trying.
I know this comment has strayed a bit off topic, but I just have to say, I find it deliciously hypocritical that Christian conservatives have the nerve to demonize Islamic fundamentalists (who are, in my opinion, currently one of the top 5 threats to liberty in the world) when they THEMSELVES do the same shit with their own religion. They just aren't as aggressive with their devious attempts to control people who want nothing to do with them.