I tried hard to push XMPP back in its day. Little success sadly, that was when IM was going out of style in favor of SMS. I kept using Trillian and watching as more and more contacts went offline never to return. Then Google announced they were killing their XMPP gateway and that was a nail in the coffin.

The bigger problem with XMPP was varying support of various XEPs leading to an uneven user experience with mismatched clients. That in itself was fixable, and not a problem for people like us, but it became a problem when trying to get 'normies' interested. Tell someone like us 'you can't video chat that guy, his client doesn't have calling capability' and that makes perfect sense. Tell an average person that, and they hear 'this system sucks and I can't count on it to do what I want, I should stop using it'. Then they go on Discord or iMessage or whatever, and it works right the first time every time, and they stay.

And therein lies the real problem. You and I can wax poetic about the pros and cons of this or that system and its security, but if I can't get my non-cryptohead friends to use it, then it's worthless.
And THAT is why Signal succeeded and XMPP failed. Because it's dead fucking simple to set up. Download the app, punch in the SMS security code, and you're online. Questions like 'choose which client software you want' or 'pick which instance you want to sign up with' kill adoption for average non-techie people. They say 'I don't know what to choose, I don't want to choose wrong and cause a bigger problem, so I'll just not choose and close this'.