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.
I noticed the same thing. There is an open issue on the Lemmy backend GitHub about broken federation in the 0.19 release here
Some instances are running older 0.18 versions which don't seem to be affected. Restarting the processes every few hours might help.