The problem is two fold. Healing in this game is really not entertaining (and there are very few real healers) for the average FPS player and is simultaneously, essentially, mandatory for a teams success. Combine that with a massive, massive amount of DPS mains and a game system that hugely punishes poor play and doesn't reward extremely good play, and at the same time there are SO FEW viable characters in this game, designed around switching.

-and you end up with the current situation. A mega toxic meta for the vast majority of players. It's a problem of contribution. Unless you are a great player you can't really contribute enough to win a game that the other players on your team don't deserve to win. However, if you're a bad player and you take the slot of a core character and you under perform in that role, you have massive, massive negative impact on your team. OW is basically predicated on the "average performance" band of a player being the ideal and not rewarding above average play as well as other multiplayer games. Meaning a lot of DPS players get very angry when they are legitimately doing very well and still losing.

OW is a very well designed game for co-ordinated, engaged players. But when you have a dysfunctional team of decent players against a happy team of lesser players, the better players just get massacred.