It started getting popular years ago and that's when me an my friends switched to it too (back when I didn't know shit about privacy). You gotta keep in mind the alternatives back then were Skype, which was meant for 1 to 1 calls, had shit audio quality and issues all the time and TeamSpeak, which was complicated because you needed a server (we were kids, we only knew what a server was from Minecraft) and had a text chat that was only a small part of the bottom of the window that was full of connected and disconnected messages, so I actually didn't even know you could write in that. TeamSpeak's interface also isn't exactly good-looking or very intuitive. Then came Discord, you could create a server for you and your friends for free, you saw who of your friends was online and playing what, you could see when someone was in a voice channel and could just join, you had multiple text chats where you could easily send a link or memes while playing and you could easily share your screen with the others. It was a major improvement over the other two. I know that it sucks from a privacy standpoint but there's good reasons why people started using it.

COVID got people used to video/audio communication, then the other platforms enshitified while discord remained as shit as they always were.

Because that's where people are?

How familiar are you with IRC?

I was told by someone that IRC is kind of what discord is built on. Maybe the answer is someone in that relation, if what i was told is accurate or not