Simply: XMPP is a protocol, and non-tech people don't know "protocols", they know "apps", at best.
Plus XMPP has challenges (and I've used it since about 2000, on my phone in 2009).
E2E is possible, but problematic (in that it's not simply just "on").
Even worse, none of the apps look polished...it's all clumsy, there's no one app on all OS's. And the names, FFS us geeks need to get a fucking clue.
And I use XMPP every day on my phone and laptop.