But we're talking about reasons people ended up choosing slack over something like IRC (or, really, any open protocol instant messaging specification).
The grand-parent comment raises good points, people want these things, being connected and reachable while not being connected to the server with any client and having a context later on. These are problems that can be solved but nobody has put effort into making a sexy product to do it. (and monetising that kind of product might be troublesome)
Slack is a closed-source for-profit SaaS, ruled by a single benevolent company.
IRC is an open protocol-specification with a multitude of clients, servers and networks.
You couldn’t find better apples and oranges for comparison if you tried.