I think there are three reasons why people don't play tank classes:

1. You need to understand dungeons before you enter. You need to know boss mechanics and trash pulls. This isn't hard, but people are really lazy.
2. You cannot zone out and follow the crowd while playing a tank (except ct :P)
3. People have high expectations for tanks, if you die or make a mistake people freak out because it often wipes the party. Meanwhile DPS can die 2-3 times per fight and no one says anything. Much less stressful.