I’m sure there were a number of different reasons that changed over time, since at one point people loved to stack tanks and later in the game when there were restrictions nobody wanted to play them. Well, I still played them and the fast queues were awesome.

Since I was one of the few, you would need to either ask other people who didn’t want to. I’m sure that Blizzard collected feedback in some way about the reasons nobody else wanted to be the tank. Maybe because tanks are slow and frequently tied the objectives while the DPS got to zip around and are faster paced with more action based abilities. DVa had some action based abilities, but couldn’t stray too far from the objective.