There was a lot of pro-Ron Paul content on reddit early on. Then there was a large and very active Donald Trump subreddit that reddit outright banned. I think

There was a lot of pro-Ron Paul content on reddit early on. Then there was a large and very active Donald Trump subreddit that reddit outright banned. I think one of the key reasons subreddits lean left is the very active use of banning. Despite having a doctorate in political science, I was banned from /r/scotus because at least one mod didn't like my politics. During one discussion in /r/AskSocialScience, I had my flair stripped from me by the mods because they didn't like my answers, even though I was the only person in the conversation citing sources. I moderate a large subreddit myself (300k+ subs) and I am well aware of how important the banhammer is to keep discussions civil, trolls out, and brigading held at bay. But for most subreddits that even remotely touch politics, bans really only seem to skew one way, ensuring that non-liberal viewpoints are scarce at best.