Google killed XMPP momentum. And while Matrix has many issues it needs to figure out, especially the development being almost exclusively supported by a for-profit company, they seem to slowly (very slowly) work towards more independence.
Matrix did some things right. Going with JSON spec instead of XML, having Element as uniform cross-platform client, offering bridges as a way to stay connected with your family and friends without needing to convince them to move (XMPP offers transports, but they function entirely differently) and offering end-to-end encryption by default.
XMPP in true open source fashion doesn't have any uniformity from user perspective. Different ways to do the same thing on different clients, different clients on different platforms. That is a benefit for a savvy tech nerd, but it's a huge inconvenience for a non-techie family member or friend.