To me it feels like the open standards never really kept up with the times, which in a way paved the way to walled gardens by the giants (and, in case of Slack, not so giants) to fill the gap which the open standards refused to fill.
I mean, sure IRC is awesome, text-only, channels, but emojis are limited to ASCII, inline GIFs are non-existent, and file-sharing (and/or storage) is rather clunky from what I remember from the old times.
And then we have XMPP which doesn't support things like video or voice calling (or at least it didn't last time I checked) and it's not very surprising that companies just capitalize on that and make their own platforms.
Don't take me wrong though... it creeps me out how the company where I work trusts Slack with reams and reams of confidential documents, but I guess it's convenient to an extent which no other platform is.