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.
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.
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.