Because they took OW1, changed very little, made it a lot more microtransactiony, didn't release a promised co-op mode that basically was the reasoning to release OW2. I'm sure there's more but that's about it.
I don't like some of the micro transaction changes they've made, but as a competitive-focused OW player, 5v5 was for sure a great move when it comes to game balance and the overall feel/pace of competitive matches.
Wish they wouldn't put heroes in the battle pass though. That is stupid as fuck.