Including on boarding, it beats slack in it's core strength.
The thing is discord has a stigma for "gaming" and slack for "work", and I can't get people to switch for that reason.
It's so dumb.
Have you looked at discord's homepage, they specifically say its "Free Voice and Text Chat for Gamers"
It's easier to manage your presence on slack than it is discord, because discord offer no assistance to those who handle multiple accounts or identities.
As with other competitors, teamspeak solved this, irc doesn't have this problem at all. Skype does, doubt they will do anything about it, Microsoft have made another solution for team management
There are many examples of inferior products winning due to marketing.
Windows is one of them, x86-64 is another, AWS is a more modern example. (people are still very much on the hype train here though)
Edit: I knew people wouldn't like me pointing at AWS;
Inferior to what and by whose standards? Itanium?