Discord got big in online gaming because they offered a VOIP and text chat browser cliemt. Just copy or type the short link and you're in in a minute. They also did free hosting which was huge.

Compared to Teamspeak or Ventrilo, literally just eliminating the steps of downloading a client, installing it, and typing in an IP address caused them to explode overnight. Also you could "host" without changing router settings (most kids/students have to ask their parents or jump through hoops for this).

Technically there was stuff like Skype but that never had the convenient team speak style chat rooms to drop in and out of freely.

Within months of suddenly getting popular, discord had a huge userbase that everybody was using already, and that momentum got us to the point where in some aspects its even replacing the role of wiki's and forums even though its terrible at it.