One thing you have to remember with Reddit (and Facebook & co.) is that they're international. I think for Reddit about half the audience is American. A small subset of those are willing to leave Reddit for political problem, say, 5%.

Even though the rest of the world is literally flooded with US political news (less so now that Trump is gone), most of the rest of the world doesn't really care about those things Americans are super passionate about.

Reddit can remain huge even if half its US userbase leaves it.

This point is even more salient for Facebook outrage, BTW.