It’s faster and it’s not Synapse. I could serve hundreds of people on a single Pi while I would need to order a VPS with 4GB RAM to serve the same amount of people. I know there’s better server software out there, but it’s nowhere near Synapse. XMPP simply doesn’t care, clients and servers are well built and almost every client uses OMEMO and honestly I had a lot of decryption errors on Matrix and if you used something else than matrix.org you’d be screwed. It’s simply just better, because it’s faster and has a bigger ecosystem. The only thing that’s not cool about XMPP is that the federated userbase is kinda small. The biggest non-federated XMPP server is WhatsApp and that’s kinda sad. Also the protocol is nice, because most clients keep a socket open to listen for new messages and this is especially nice in the college WiFi environment some of my friends are in where a timer is set after bedtime which would wait until all sockets are closed which doesn’t include XMPP so messaging with my friends after bedtime is still possible.