In a nutshell, one of key aspects of the reddit system is that an upvote arrow affects everybody equally, so the majorities affect whether something is negative or positively rated. When you take that into account for where and whether you show a post, the net effect is that at scale you pretty much guarantee that the majority within a given community can suppress the minority. The majority may not even have to necessarily try; it's something built in to the system, even before we get into matters of people deliberately trying to game the system.
Systems like Slashdot, HN, and Reddit try to fix that either with human moderation or a variety of heuristics to try to minimize the impact ("that looks like a voting ring, let's make its votes worth less", etc.), but another alternative is to make the upvotes not be universal anymore. Instead of an upvote meaning "I think everybody should see this more", it could be rewritten to mean "I think I want to see more of this poster". It's more math, but we have bigger computers now. As a side effect, it's robust against a lot more attacks, though I don't guarantee it wouldn't have other weaknesses.
What's to stop other groups from starting more focused subreddits? I understand what you're saying /r/Politics has a clearly left lean to it, but /r/Conservative seems to be relatively healthy (from my occasional peek in).
Even something like The_Donald still survives, and I'd dare to say it's not the content, but its constant attempt at stunts and controversy that's causing the most ill will towards it. I know we've seen hate subreddit's deleted, but I feel that's largely understandable when the very goal of the subreddit is toxicity toward a particular group.
I guess the fun with this is that the biggest items on Reddit are going to be the ones that have the biggest population interested in them, but I don't know if it's necessarily hard to create a sub group, it just won't make it to the front page. I'm subbed to a ton of subreddits that meet that criteria.
Nothing, and as you point out, many do. At the same time, default is a powerful thing.
I'm a mod for /r/cscareerquestions, and while we're ostensibly non-political, the fact that we try to keep a minimum bar of 'professionalism' means having norms that are somewhat left-leaning. As an example of what I mean, "it's less important for women to have strong careers than men" is a very common opinion among social conservatives, but we'll ban posters who say things like that because, well, it's sexist and unprofessional. This kind of thing isn't limited solely to right-leaning opinions, but it doesn't tend to be that way more than if it for left-leaning opinions, I think.
Consider these: r/news
r/worldnews
r/neutralnews
r/fullnews
r/qualitynews
r/worldevents
r/inthenews
r/anythingoesnews
Eventually the subs become more and more specific and devolve into echo chambers. For whatever reasons, this seems common among philosophy/opinion subs and doesn't seem nearly as prevalent among topical subs.
Don’t like downvotes? Try a different subreddit. There is always a more welcoming and engaging subreddit just around the corner.
Personally I use Reddit for amusing and funny subreddits and so the political bullshit is never my problem. And you can have the same today for the low, low cost of not giving a shit about echo chamber groupthink.
Bad actors get punished. The gamergate and incel subreddits were taken down because they incited harassment. But to use your other example of r/southafrica, blame the 90% of the country that isnt on Reddit for allowing that subreddit to descend into groupthink. Their country, their subreddit, their responsibility. Not Reddit’s.