There might be a mix of explanations. Some are just nuts. Others were brainwashed at some point in their lives, or maybe just blew their brains out with drugs. My guess, though, is that the leading cause of conservatism is chip on the shoulder syndrome. A lot of people feel that life gave them a bad deal, and are filled with envy and hatred of those they consider more fortunate. Still others know they got a better deal than they deserved, and feel obliged to disdain and hate those less lucky.
Funny that liberals don't seem to suffer any of these problems.