At this point, familiarity and nostalgia. In the heyday of IRC it was a good way to get help (or flamed) for questions that couldn’t be found anywhere else online. As a former IRC user I’ve never used matrix and have only heard of it on social media like lemmy.

As a programmer I like IRC because it has a simple protocol. Writing a basic irc bot is pretty easy if your standard library supports sockets and string manipulation. I was doing this sort of thing in the late '00s but things have changed a lot in 15 years.