While that indeed is an advantage, it's not why they're dominant, and I'm not sure it's simply a question of funding either.
Let's be honest, compared to IRC, Discord and Slack are very easy to use. All it takes to join a server is clicking a link. IRC protocol is just as old as me, and I, to this day, remember the struggle as a kid of joining empty channels before finally understanding the concept of networks. Back in the day, you'd need a special friend with ZNC to be the cool kid. Since then, audience has changed dramatically and most people don't even know what "protocol" means.
IRC just doesn't offer what it takes to get everyone on board.