Signal is the world’s most widely used truly private messaging app, and our cryptographic technologies provide extra layers of privacy beyond the Signal app itself. Since launching in 2013, the Signal Protocol—our end-to-end encryption technology—has become the de facto standard for private commu...
XMPP is the IETF Internet Standard while Matrix is just another custom IM protocol managed by a venture capital funded startup which keeps losing money.
I don’t disagree with that statement; however, that doesn’t make it something the general public will embrace. Its mess of extensions are top little too late. That ship has long sailed. And I say this as someone that prefers using XMPP for 1:1 chats