Probably because the game is pretty simple, and the meta is quite strict.
People got a good understanding of the entire game, within weeks, and that understanding was available to even the most casual player. It was easy to see which characters were useless, and which were useful, and the manner in which the game allows you a lot of flexibility with what you pick, combined with the fact that many of the picks are effectively bad choices.
The problem with that is that you can switch character any time, so what would have just been a moment of frustration as players realise their team is suboptimal, becomes more frustrating because the players believe they can will their team to switch over the course of the game.
Additionally, the game itself is incredibly team orientated. It's difficult to get anything done on your own, even compared to traditional mobas, because the game is effectively a constant 5 vs 5 team fight. Many abilities, and in some cases, whole characters, only play with any degree of effectiveness if paired with appropriate team mates. Again, this makes it more frustrating if your team mates aren't in sync with you.
Alternatively, if players do listen to the people screaming at them, and switch character, they're often playing a character they don't like to play, just for the sake of winning. That isn't typically a whole lot of fun, and when you're playing for function, rather than enjoyment like that, I'd imagine it's more likely you'll be irritated when things don't come together. If you make me play Lucio for 10 minutes, and we still lose, I'm going to go away from that a little bitter about the experience.
Together, these features lead to a perfect storm for player frustration.
In my experience, Paladin's is much better for its community. Most likely because if someone picks the a bad character, you can't shout at them for the rest of the game to switch, what's done is done. Plus, they can also adapt with item purchases in the game, while still being able to stick with the characters that they like to play.