1. Old people do not use it.
2. Very responsive, searching for previous messages takes seconds.
3. I remember back in '17, most other VOIP and especially video calling services sucked. Discord made gaming with your non-techie friends with a crappy laptop plug & play.
4. Lack of censorship (at the beginning).
5. Good UI when it was released compared to its competitors.
6. Gaming Centered Nature back when that was not common and/or alternatives were complicated and clunky (TeamSpeak).
7. Very good integration of GIFs, meme emojis, and the like.
8. Old people do not use it.