Tanking makes sense in open queue, because stacking tanks and healers makes the comp broken. Everything works in harmony both defensively and offensively.
But when there are 3 different roles, tanking becomes more of a responsibility.
In role queue, Tanks not only need to absorb all the damage and cooldowns, but they also rely more on their teammates for doing their job.
While on the other hand, DPS and Support becomes more of a laid-back experience, which pushes the players to queue for the easier and more relaxed roles.