Discord is centralized, while on IRC you have tons of different servers and you can even run your own.
IRC is ubiquitous and has tons of different clients, so you could just as well connect to a server on a 30 year old computer you found in your basement, as opposed to Discord which only has one client that requires running a web browser.
On IRC you can also have people come and go in a channel, which in my opinion makes it much more fun, though you also have the option to make your channel invisible. You can easily /list and search for stuff that you like or /whois anybody and join any channels that look interesting, rather than googling around randomly.
It's definitely not perfect, and I wish there were people interested in improving it, but being so ubiquitous and decentralized is what will make it stick around and outlive everything else.