Simple: normal, everyday people are boring. TV shows want to make money, and to do so they have to get people to watch them. Shocking or dramatic content is one of the easiest methods of getting people to tune in to a show, and nutjobs provide lots of it for free. Hence they'll get lots of airtime while normal people are rarely if ever shown.

Yes, it's exploitive and probably very demeaning, but to some there's no difference between famous and infamous.