I’ve been an XMPP user for decades. I use it daily to talk to all of my friends and most of my immediate family, so I’m certainly an advocate for it.
I think in the simplest terms Matrix gets mentioned more often because it’s newer and it’s development has largely been driven by a corporation with significant funding. XMPP had similar corporate sponsors years ago, but a lot of them moved on. The XMPP community isn’t gone, though, and XMPP development has continued.
As others have said, it’s true that XMPP had some difficult issues in the past with various clients only supporting certain extensions (XEPs), which meant what worked on one client may not work on another. This problem isn’t nearly as much of an issue nowadays, but it was frustrating in the past. There are official compliance suites that clients and servers can use to ensure they meet the base features for a particular purpose, so that has helped in that regard.
To me, XMPP is preferable because it’s a messaging protocol, not a giant graph database of all social interactions. I don’t need or want that. The main XMPP server software does handle message history that is synced across all devices, but this is often user configurable and none of that information is needed by or shared with other servers. MUCs are not replicated across the network, but only exist on the server that hosts them. While this does mean that server is a single point of failure, it’s much simpler to operate and, at least in my opinion, is better from a privacy perspective.
Traditionally XMPP, albeit with propriety extensions, has been used as the basis for a large number of messaging platforms. Google Talk was XMPP. Facebook Messenger was XMPP. WhatsApp still is XMPP, as far as I know. Kik is also XMPP. Google and Facebook’s platforms may still use XMPP, but I don’t know for sure. It certainly works as a messaging platform for lots of users, but without all the VC money, development is pretty much only done by volunteers or client developers that offer paid builds of their clients (like Conversations on Android).