Mostly because they don't understand it. And when people don't understand something they make fun of it and attack it. There are those who do the research and try to learn more about it, but they're few and far between. We have to rely on ourselves to get the positive images out there among society that mental health isn't some crazy psych-ward boogeyman that's gonna get ya.

NIMH http://www.nimh.nih.gov/ needs support, because they are trying.