If have I had to guess… a combination of well meaning rules built up over time that no one actually thought out, chronically online weirdos corrupted by the little bit of power afforded by moderating (which seems common in online communities) and cabals of state actors who moderate multiple subreddits in order to control and direct the narrative.
On the other hand, with how popular reddit has become and with how many bots are out there, it might be worse without the rules.
Anti-China comments aren’t the same thing as anti-Chinese comments.