Normal people won't tolerate being on the same forum with huge assholes. Especially assholes who are bigoted, rather than just generally angry/jerk-ish. The far right happens to contain a lot of this kind of person.

The far left has people who are also very angry, but they're generally not as bigoted. The demographic they're most angry at is rich people, which is punching up instead of down at least.

And most of the rhetoric there simply isn't as vile. It's stuff like, "take rich people's assets so we can redistribute it equally". I may not agree with seizing wealthy people's stuff, but that's nowhere near as offensive as "kick out all the gay/non-white people".