Xmpp is old and has no traction. Matrix is new and there are many people believing in it. There is a lot of money put on matrix. A lot of people want matrix to succeed. Especially companies, agencies and governments love matrix. Jumping on a train that already moves forward is easier than trying to push a standing train.
With xmpp, or signal I've got all my messages on my device. Distributing the info to other devices is difficult. With matrix everything sits on the server and distributes the info to the clients. That's like my file cloud, or my photo cloud or my music server, or my document server. Everything is saved centrally on a server and all is independent of the consumer device. I can use multiple devices and everything sits on the server. That's great for me as a user, it's easy.
Xmpp is scattered which is great on one hand but matrix development is moving very fast. Xmpp can't compete with that.
What's the advantage of xmpp over signal for the end user?