Slack gives you a unified chat stream and a simple, buttery-smooth onboarding process for the tech-naifs. Jabber does neither of these. I'm not fond of Slack in particular, as it's ridiculously expensive and the main chat window is too widely spaced, even with mods.

I use jabber with a friend, and frequently messages go to his other laptop, which he doesn't see until he logs onto it. Same in reverse. Similarly, history isn't unified. You can get around these problems by having your own centralised jabber chat client (like bitlbee), but that's a technician's answer, not an answer for the general public.