Running a Discord server has no cost associated with it. When I ran a TeamSpeak server, the costs were reasonably high, and my server had only a maximum of 50 voice slots at a time; that's not enormous in terms of an active group.
But Discord also has a fully functional web client, whereas Teamspeak does not. Skype does, but who the hell uses Skype any more, except in a business context (and even there, it's being replaced with Teams or other tools).
Discord is just more convenient to pick up and run with, without the hassle of setting up servers, downloading and configuring clients or troubleshooting technical issues.
Discord is more convenient for creating permanent groups, LFTs and communities Baby Balthsaid: Because only jagex employees do group bosses on account of eoc being shit. Mind you even they got fired. Jmods can't PvM lol
A Jmod team still hasn't done HM rago Jmods bought Solak leeches lulYou'll get it when you deserve it.