If you look at these communities, there is basically only one feature of Discord/Slack that they use that IRC does not easily provide and that's ephemeral/mobile. If I go offline for an hour in IRC, I lose a bunch of messages unless I've done some backflips to set up some kind of archiving/replay in a separate failure domain (e.g. znc running on gcp). With chat services, the service is essentially znc for free.

The rest of the shift is explained by discord being popular for other use cases (e.g. as a twitch companion), overwhelmingly dominated by younger users. IRC isn't getting new users and over time, this means the community will migrate due to turnover. Thank god we at least have things like matrix bridges, but discord is a lost cause.