They have a <10 person team IIRC, using Discord as an interface saves a TON of money/effort/maintenance/risk, etc, as best as I understand, and lets them focus on the technical product. Remember, Discord is giving them free content legal protection by proxy, even if that's not necessarily the original intended effect I think. There's a lot to gain by riding alongside Discord as a primary interface vehicle, I personally believe.

They're good enough technically at what they do that their audience is okay with the interface that they have to use, I'd reckon (I've heard similar beefs about the UI stuff though, so it sorta makes sense).