Suspect, pure speculation, when they pivoted from being focused on gamers, switching from guild to server helped.

In the dark ages of the internet, you had to host a server in things like Ventrilo[1], which was the thing Discord tried and succeeded at disrupting. Since now you can create a "server" for talking with friends (and friends of friends) without being technical.

[1] https://en.wikipedia.org/wiki/Ventrilo