One answer to the question posed in the thread title: Ignorance.
And there are two types. Genuine ignorance: 'Oh, I didn't know that'. And willful ignorance: 'I refuse to listen'.
In a forum some time ago (Catholic Answers I think), there was one guy with whom I used to discuss evolution. He denied it. But he wasn't the type who stuck his fingers in his ears and sang la la la. He know a lot about evolution, but disagreed with it. And far from the usual claptrap of 'if we evolved from monkeys...', he put forward well thought out arguments against it. They were wrong, but at least he knew what he was talking about. And you had to know, or find out, how to refute what he presented. I learnt a lot because of that.
I miss discussions like that.