Maybe it's because the gameplay of Tanks/healers isn't "fun" for most people.

I know there are players that like the roles but there needs to be more then just being a meat shield that stands there doing puny damage and hitting aggro skills, more then a healer that sits there in back tossing on heals.

Warhammer even made healing more fun then most mmo's. As a shaman you actually needed/wanted to fight and do damage, as this built up a meter which in turn made your healing skills better and more useful. So as a healer you actually took part in combat and didn't just stand back hitting heal over and over, unless you wanted your healing to be less helpful.

Tank wise, this is where Moba games have took it ahead, it's just more fun (imo) to play tank roles in mobas, where tanks aren't merely a shield who does no damage. instead it's more about being able to disrupt/taunt, and you can still do decent damage, you don't feel "powerless" when it comes to having an impact on the battle.