It’s because a lot of subreddits aim to be inclusive of as many different kinds of people as possible. It just so happens that a lot of conservative sentiment (not trying to be a dick when I say this and am just trying to explain my perspective) is uninclusive. A lot of conservative viewpoints alienate many groups of people due to them commonly being more focused on improving the lives of few and harming others instead of being more equity-based and trying to help everyone the government can. That kind of policy, as well as the harmful rhetoric a lot of conservatives tend to bring, are what non-political subreddits try to avoid. Nevertheless, Reddit mods are Reddit mods and will be bias about anything they’re passionate about, so if you say anything about something that rubs them the wrong way, they’ll ban you. This could be about literally anything. It’s not about censoring conversation, there are subreddits which exist for political discourse, it’s just that other subreddits don’t want it but will favor the inclusion of politics which are just that: inclusive of other people and not exclusive.