It has way more features than Slack and those others. Just the first things that pop into my head: up arrow to edit, code syntax highlighting, custom (animated) emotes, quick jump (like macos Spotlight), built-in Giphy search, the best and most reliable screen share and video calls (with voice channels, not just DMs/groups), unbeatable voice isolation and echo cancellation (better than Zoom!), very good built-in webhook support and likely the most powerful bot API of any platform.
Maybe some platforms have some of these features, but I'm pretty sure no one platform has even half. That's why Discord won.
Literally everything except video and voice was introduced with slack. Video and voice was horrible and only got really good when they already won the 'battle'. The API and Hooks were even borrowed from slack initially and only later extended.
I am not even a fan of slack and have no need for a group IM like this at all. However discord was everything but revolutionary.
Nobody ever said any of the features features were revolutionary. I don't care what they were like in the early days, or even last week. Right now, at this moment, Discord has all of these features and the best implementations of most of them them at that. And again, it's free.
Like it or not (and fwiw, I don't - trusting most of my communications to a highly over-valued VC-funded startup sounds like a recipe for disaster), Discord is damn good software!
Maybe some platforms have some of these features, but I'm pretty sure no one platform has even half. That's why Discord won.