Earlier quoted context omitted.
IRC isn't everywhere. Even a large channel like freenode's #node.js has maybe 15 regulars, myself included. Meanwhile, Elm's Slack channel has more regulars than #node.js. IRC is kept alive by a small holdout. I'm one of those people, but it's no mystery to me why nobody else uses it when I'm paying irccloud.com or running weechat-ncurses on a VPS just for basic features everyone expects from a communication tool.
“Large” means #rust on moznet, which normally has around a thousand people in it, and has scores of regulars actively chatting.
[1] I'm curious as to whether that IRC channel is the only "blessed" Rust chat out there. The problem with other languages is that the community makes a Slack for them even though there's an IRC channel for them already and most new users start only going to the Slack.