Maybe I'm just old and grumpy, but am I the only one who finds the UX to be atrocious? Everything seems to a nondescript icon that rarely represents what it's supposed to do, nearly everything is flat so it's hard to tell what you can actually click on and whats decoration. Everytime I use it feels like an exercise in mental gymnastics just to navigate it.
I'm 40 and grew up on mIRC, Discord feels a natural evolution with all the new features.
I also feel like they capitalised on something specific that gamers wanted (chat and voice calls with low latency). Skype call quality is very much hit or miss, and while Mumble is great not everyone has the expertise in setting up a server.
A common person can learn. Also the audience (mostly gamers) know how to do stuff with their computers.
It also doesn't require a bouncer just so you can go from mobile to PC to tablet without issue.
The nice UX makes it easy for everyone to use, and the community aspect is appealing.
IRC isn't very easy to use for the common person.