Discord attracted gamers because they could sync up during games, raids, etc (and on top of that you can advertise it through your status too). Otherwise it's Slack for people who don't want to feel like they're at work, in line with the spirit of the last few generations of gacha games (boost this server by paying, get more emojis for yourself by paying, etc)
I agree that the text is the major activity going on at any given moment because text remains a good asynchronous repository, but if they hadn't basically integrated mumble/teamspeak to slack I'm not sure the glitter would have been enough to stick, because gamers (and some groups of discussions) definitely do rely on those voice lobbies when actually being present.
Join any big Discord gaming group and I guarantee you'll see more people chatting via text than using the audio functionality.