Moderated media leans left. At least some of the reason it ends up that way is that many of the people who violate incredibly reasonable rules are conservative. Certain groups of hard-right people will say some incredibly bigoted shit that's absolutely out of line and makes it impossible to have a civilized conversation, then they complain about getting banned and drag a bunch of moderately-more-reasonable people with them when they leave. Once those people leave, normal everyday non-asshole conservatives realize the platform has less conservative content and leave in search of spaces that they feel respect their viewpoints more. In some cases entire topic-groups get banned (/r/the_donald is a good example) for legitimate reasons that frequently involve a small extremely-active group of members, and the rest of the members will also leave the platform because all they see is that a group they were part of got banned.

People who lean to the left tend to believe that it's bad to do some of the things that get you justifiably banned (such as intentionally using language that demeans people based on immutable traits). Because of this, it's much easier for them to avoid being deplatformed.