Discord often moves too fast for important updates to be posted. You can do an announcement tab, but it's just easier to keep up with stuff in the OOC. Can't tell you how many times I've come to an active discord with 400+ messages to read, only to see there's virtually nothing important going on and it's just memes and shitposting wall to wall. 

And because I never tire of forcing my danganronpa RP awkwardly into every conversation, I found when the size of the cast shrunk down to 4 players, keeping the discord up was a fool's errand. No one used it, and it was much easier to type out long announcements without a 2000 character limit. Not only that, but OOC posts bump your RP. So randos can see your player's banter and don't have to wonder how active things are in the discord. It's better for getting more players.