fancy IRC
IRC was already "caveman playing with sticks and pebbles" a decade before discord became a thing. It's really not a good point of comparison and questioning.
Discord became popular for one simple reason: anyone could make a server, share it with a crossplatform link, and others could then try out that link without installing anything. In other words, it became popular because it literally copied Slack and because the Skype era was atrociously bad customization and ease of use-wise compared to the preceding.