First, IRC isn't easy for newbies. I tried to adopt it at the first tech company I worked for, and our support folks had a lot of trouble getting the basics down. There could be a client that makes things easier, but I've never seen one as easy as Slack.

Second, IRC alone doesn't provide a bunch of features that everyone expects nowadays. You have to host or pay for a bouncer if you want to see what was said when you were offline, for example. Gotta use a 3rd party service for push notifications on iOS. Again, there's no reason why this couldn't exist, but it's another product, not just IRC.