Fred99:
Because social conservatism is a religion.
There's no other rational explanation for why some people would want to impose their conservative values on everybody else, legislated - and therefore able to be legally enforced, if resisted then ultimately through initiating the use of force - without some belief system that they have which grants them the moral right to force others what to do what they tell them to do. They are much the same as hard-line communists - which should also be considered a religion IMO.
I'm not sure that communism has ever really sort to explain the reason for the existence of the universe.