Bottom line, in roulette, Tank and Healer are just not as fun as DPS (subjective, but hear me out).
Tank has to pay attention and otherwise generally set the pace. There is an expectation overall to know mechanics and avoid or mitigate damage/needless death because if you (tank) die, everyone may die.
Healer similarly has some need to pay attention to tank busters/general mechanics in order to keep the party alive. Your job also becomes harder if the other players are terrible. Obviously your death significantly imperils the party.
This is not to say EITHER tank or healer is necessarily hard...it's just that DPS is that much easier to do. Even if you die, the party can generally trudge forth. You really only affect the speed the dungeon is cleared unless there are DPS/enrage checks. This lack of pressure is very appealing.