And yes, even on non-political subreddits politics has taken root. Any time there is a political photo it turns into a full blown political subreddit.
Conservatives are a minority in America, so it makes sense their opinions would be unpopular by virtue of that.
There's nothing that's not political, every aspect of life is political, how we distribute resources, interact with our neighbors, basically everything is political and sidelining political discussion is itself a political act which benefits whoever is currently wielding power.
[1] https://splinternews.com/here-is-the-final-popular-vote-coun...
The Senate holds a conservative majority because it's structured to represent land rather than people, and conservatives are occasionally able to take the House because they use districting to deprive the majority of voters of the majority of representation.
I think it's one thing to say reddit is mostly people'd be leftists, which I agree with, and another to say that conservative opinions are explicitly banned. I think conservative opinions are unpopular and the imbalance makes reddit a poor place to understand the politics of the median person in the US, but non-leftists are allowed.
[1] https://www.reddit.com/r/politics/comments/fdpixv/city_of_mi...
[2] https://www.reddit.com/r/politics/comments/f8cdys/bernie_san...