Same thing that got us the Ft. Hood shootings: these people fear for their careers in a PC world. Making an accusation that places the accuser into a protected group, like homosexuals or mulsims, is a high rish move.
People have to feed their families, and can't if they are blackballed for non-PC behavior in their chosen field.
At some point, though, “I’m afraid of getting fired” just doesn’t cut it. The PC crap is enabled to continue when people forsake moral courage in favor of having an easy life.