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.
and all the teenagers in all the high-schools who otherwise had no affinity with computers whatsoever.
/slap
But that was 20 years ago, of course. You can't compare the technical prowess of a random kid back then with a current well-paid software professional with access to portable hardware 100s of times more powerful.
There's a reason Mozilla moved away from IRC, and it wasn't that "their engineers couldn't figure out IRC".
It is easy to dismiss something as "just a chat product" but then again, it is incredibly hard to make a product that is so easy to use that everyone can use. Reminds me of that comment about Dropbox from years ago https://news.ycombinator.com/item?id=9224