Consider two possible statements:
1. Hitler wasn't that bad.
2. Stalin wasn't that bad.
I think, for most people, the first provokes a much more extreme reaction. Both were objectively terrible human beings, but defending Hitler is seen as far more extreme than defending Stalin.
This has two effects:
Firstly, far-right people are continually kicked out of communities. Far-left people are not. So any new unmoderated community is going to attract these "refugees"
Secondly, nobody notices or cares when a community goes far-left. But its far more noticeable when a community goes far-right.