They are just so unpleasant. I remember when I was a baby leftist trying to learn more, and feeling so distressed in every interaction I had with /r/communism, that it probably delayed my progress by a few months. I thought, these are not people I want to meet or interact with in real life.
Reading and learning about how communists like Huey P Newton or Fidel interact with people, always with respect, always with the idea that they truly want what's best for you and humanity, and seek to enlighten rather than alienate, you realize that /r/communism is trying their best to do the exact opposite and alienate every single person they can. This is why reddit lets them exist, but bans other genuine communist subs.
Their ideal is the Gonzaloite terrorist killing peasants for not being revolutionary enough.