They rolled out a catastrophically bad redesign of the site, resulting in a mass exodus of users directly to Reddit
Digg fundamentally changed the way their site worked to highlight content from power users.
When you become huge, inertia is incredibly strong.
It's all about enthusiasts and early adopters (who are very fickle) and late adopters and laggards (who are very much the opposite of fickle). Crossing the chasm. Reddit has crossed the chasm.
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.