And because bigots feed on fear and control, control of someone's life to the point someone actually pretends to be something one isn't.
Going public deprives them of that food. Not being feared and not controlling someone ''different'' removes their means of projecting their own problems, their own shortcomings on to others. Like high school bullies, there's always a reason someone becomes a bully, lack of self confidence etc. As soon as their victim stops being afraid and steps up against them they loose all their power. They actually become the mocked ones. A powerless bully is a pathetic sight. Just like a powerless bigot.
That's why I love gays going public. I feed on seeing bigots squirm