accessibility is usually pretty horrendous on most matrix clients, none i could find have full, proper reduced motion support, which i need to properly use pretty much any piece of software. discord is far from perfect in this regard, and getting worse and worse, but still miles ahead of most chat apps i've tried, especially proprietary ones. I have barely used revolt, and in my memory it's far worse than discord, and i have yet to figure out how xmpp works...
none of my friends are on it. while i could convince some to move, here discord is already a quite "niche" chat service, and as a student, i'm more or less required to be on any chat service whatever group project i need to do is organised. same with friend groups, i'm usually not a "group leader", and join in friend groups that already exist, so it's much harder to insist on moving anyone over to a new thing no one but me has ever used before
ux isn't great. it's improving, but i've had quite a few "key exchange" issues with matrix especially... Also, i found most clients quite messy, but that's more to personal taste!