I don't think the reason people don't stay as tanks or healers has anything to do with how fun the role plays in terms of actual play style. I think it's more down to us, the community, who throw flame at a tank if they loose agro for even a second or at a healer if they miss just 1 heal. I started ARR as healer main but after the salt I received in a particularly challenging Garuda run I vowed never to play again. We are to blame not SE