One reason is that installing your own Matrix server is more difficult than installing your own Snikket/XMPP server.

For example, one way of installing Matrix (and the most easy in my opinion) is this one: https://github.com/spantaleev/matrix-docker-ansible-deploy

Compared to installing a Snikket server this is much more complex (in my opinion).

Another reason is that the Matrix protocol uses much more meta data than the XMPP protocol (I am not an expert on this, but I am reporting what I hear from other people). Meta data are not encrypted, they are stored on the server, and they are replicated across the federated servers. This might be a problem of privacy/security, but it also increases the bandwidth, storage and in general, the resources that are needed.

So, Matrix may have more features than XMPP, but XMPP is leaner and more efficient.

But I am not against giving it a try too, if there are people willing to do it.

Dashamir