Because summaries like this still stand up as of today:
https://news.ycombinator.com/item?id=8998290
> XMPP is great for what it was designed for. It doesn't work well with mobile, high packet loss & high latency connections. XMPP is talkative and bandwidth intensive - bad for limited data/battery applications. It also wasn't designed for today's 1 person multiple devices reality. Most XMPP servers let you log in multiple times but messages don't sync between clients and sometimes get delivered to the client the user isnt currently in front of.
> Also, sending files over XMPP has pretty much always sucked - there are a bunch of incompatible ways to do it and it's always been hit and miss depending on which client your chat partner was using, network topography, etc.
Also, overload of XEPs doesn't help the ecosystem. Too many optional extensions hinders interop. People expect more features in modern chat experiences than what XMPP was designed for, and that's what XEPs have tried to fix as a bandaid, with mixed results.
On top of that, the technology choices are par of the course for the time it was designed, and nowadays there are arguably better things. Devs are naturally driven to choose tech that makes their work nicer, if they enjoy it, so that means more stuff gets done for the newer platform, in this case. As an example, coincidentally, another HN post today was touching on one of those points - the need for a very advanced XML parser, as a typical one apparently wouldn't be enough:
https://news.ycombinator.com/item?id=36930196