mumble (which displaced ventrilo/teamspeak as the previous dominant gaming chat clients) was already displaced by curse voice, which then got displaced by discord.

discord was selling itself as "the good guys" who would never do something scummy, and targetted the market as a slack for gamers, and right before launch, they sold out. but it was already "too big" to get people to change client. and now it's growing even more because people are looking at it as an alternative platform to facebook groups - now that facebook is fucking up groups.