I'm more concerned with the delays and resource usage, personally.
Then why isn’t it?!
The same reason any open-source project that's community driven isn't as polished as it could be; there's only so many resources and time available to improve things, and much of it is reliant on individuals with the needed skillset taking it upon themselves to improve something that bothers them.
Apparently not enough XMPP developers use iOS for them to focus on polishing up that experience.