Personally, I think what happened was that GChat became popular, and killed all the alternatives, and then basically stagnated and confused the shit out of its users through hangouts etc.

Slack was one of the first products that realized that since GChat had gone, there was a huge gap in the market.

I’m hard pressed to think of any area where they were beyond predecessors in either ease of use or features (I can’t remember if persistent chat rooms were available in all the other messengers) beyond those little custom emoji things.