The network effect is obviously a big part of it, but it’s also that IRC is extremely barebones and difficult to use even for messages.

I’m quite a techy person and I even wrote my own frontend to Discord a while ago, but I’ve struggled with things like bouncers and authentication to the point where it’s just easier to set up a bridge to Discord and use that when I need to use IRC, and then I miss features like editing and profile pictures (there are proposals to add these things to IRCv3 but they aren’t widely supported)

IRCCloud seems to have a good solution to some of this but is paid