I would assume it's mostly marketing. There's no entity behind IRC to push its adoption and herd the cats to a singular goal.
The lack of features that Slack/Discord specifically added to differentiate themselves from IRC are also important, but it was a lot simpler to add them to their brand new designs, than to work through having to specify them onto the IRC protocol and then get servers and clients to adopt it. You can look at the IRCv3 page linked higher in the thread to see some of the efforts, but this is 10 years later and we're still not there. :D
The lack of features that Slack/Discord specifically added to differentiate themselves from IRC are also important, but it was a lot simpler to add them to their brand new designs, than to work through having to specify them onto the IRC protocol and then get servers and clients to adopt it. You can look at the IRCv3 page linked higher in the thread to see some of the efforts, but this is 10 years later and we're still not there. :D