Is Discord popular just because it's free? Perhaps I am a bit biased because of how I enjoy accessing IRC using IRCCloud - they take away a lot of the classic challenges with IRC, like mobile support, always connected, etc.
IRC's lack of mobile clients is one thing, but features like attachments, code highlighting etc that programs like mattermost, slack and rocketchat have may be another. Also, discord is popular with young people outside technical projects already too
Those are exactly the features that were solved by using @IRCCloud for me. ( irccloud.com ) Obviously, gamers are powerful online communicators and can en-masse make a platform win or lose. Perhaps I'm just old-school :-D