4 pane comic of dolan on the left and spooderman on the right
pane 1 (dolan): cum join opensurce cummunity!
pane 2 (spooderman): shure! how joyn?
pane 3 (dolan): Here discord! (with discord logo)
pane 4 (spooderman with tears in eyes): y u do dis?
My guess: The kids who used Discord for gaming grew up, and just went with the familiar thing when starting new communities and projects.
Also, Discord did heavy marketing early on, until it carved out a network effect. So here we are.