It's not parsing performance, computers are plenty fast for text IM. It's not bandwidth, the difference between JSON and XML is negligible after compression. It's not developer ergonomics, any sane programmer is using a library that abstracts either format. It's not compatibility with the domain, as XML wins for XMPP due to namespaces.
The answer is that XML looks old. New programmers (half of all programmers have less than 5 years of experience) grew up in a world of JavaScript where XML was "legacy" since the day they arrived. In reality it's kept working fine, while the volume of software has increased around it naturally using the trendy tools. They've not looked back to understand why it was made or why it has merits, it's already got negative connotations and they're caught up in the new stuff. The new stuff has merit too, that is why a programmer is wise to respect both.
Also, what use is a stable and mature XML ecosystem when you can earn big nerd points by reinventing XPath for JSON the 5th time?
As a user, I don't care much. But my experience with XMPP is that was not as solid as other solutions, including closed source ones. I could've been issues in clients' implementation, but overall it wasn't great