Discord won because Skype couldn't keep up with ever growing gaming community and their demands and it wasn't specialized for gamers at the first place. Modern gaming is very specialized and you need a software to game efficiently with other people(communicate, coordinate, inform, message etc.)

I'm not sure how Slack is related to Discord but they seem the same for me UI wise. I always wondered why Slack didn't purchase Discord and turn it into their gaming division.

Before Discord you as a gamer could either use clunky Skype or something like outdated Ventrilo.