I like BirdDancer's first answer but I think I can adequately sum it up easier, and add one or two other thoughts. (SorryBirdDancer, I'm having trouble reading long posts like that right now.)

In my mind there are three major reasons why people don't stage protests for the merntally ill. Of course a lot of people don't go to protests for anything. That's not a judgement, a lot of people, myself included, aren't typically protest people.
1) People think "Thank God, there's one less of therm in this world."
2) People are deathly afraid that if too many of us are in one place we'll kill them all. Heck, they're afraid that each one of us is milliseconds away from turning into Jack Nicholson from The Shining.
3) People are afraid that if they take a stand against the mistreatment of the mentally ill other people will think they're mentally ill too.