I wasn't referring to things like IRC. When Slack was initially released, it was no different from Campfire and a whole string of other web-based chat systems that came and went going all the way back to the dawn of AJAX in the late 90s. Slack's improved a lot since then, with app integrations and other features, but fundamentally it wasn't any different than its predecessors. It's easy to think that Slack did something groundbreaking, or figured out the magic solution to the problem that sank its predecessors, but just like Discord, the reason Slack won is because it came along at a time when companies can raise tens or hundreds of millions of dollars to float them for years while offering a free product. Then they can upsell later, and/or commoditize their users' personal information. Those business models weren't as easy to come by in the past, so a lot of products failed. None of this is to bash Slack; it's an adequate product for what it does.

Another big thing that the current crop of winners has going for it is that cloud hosting allows applications to launch literally for free and scale quite a bit without paying much of anything in infrastructure costs. That also wasn't an option 10-20 years ago.