The network effect as seen in the other comments plays a big part, but also discord offers a useful service that really nobody else does well. there's a lot wrong with it but you can still create a community in a few clicks and you have text messages, photos, videos, gifs, voice chats, screenshare, a comprehensive permission/role system, tons of bots.. all for free and without needing to be too tech savvy, that's pretty damn cool.