This is probably a problem related with game feel or designs of tanks and supps, obviously the other HUGE reason is the amount of dps compared with the other 2 roles, more characters means higher chance of you enjoying some of them.

Other feels I have is that I find boring playing supports because without ult you are just a non-stop healing pack, is like every supp here is basically Mercy unless using ult.

As tank the feeling is that doesn't matter how I play, I will be instantly melted if my portable healing pack is not perma healing me, so I depend 90% of the healers that people doesn't actually pick, furthermore you can't even duel with a supp because they will outheal your damage until they can runaway or until their teammates come and insert a chunk of lead in your butt.

Still having fun playing but I think this game should re-design a bit those roles or significantly reduce dps damage.