Usability. I don't think Hexchat is "good enough". Slack is vastly superior from a usability perspective in my opinion. I don't have to use commands for the most part, I can customize my interface as much as I "need" to and there's no setup, also there are mobile apps that "just work".

It's almost like what OSX/iOS is to Linux. Sure Slack has a particular way of doing things, but they take care of all the hard, annoying shit you don't feel like doing.