How do you divide roles up by race, religion, sex, and sexual preference? There are some of these segments that will be unhappy if they are included as part of another segment. Splitting role up into good guys and bad guys is even more problematic. It is pretty easy to figure out who all the bad guys will be.
And why have a greater representation of other mental illnesses? Why not greater role for schizophrenics, psychotics, compulsive disorders?
Same logic.