Apple couldn't open up Facetime due to Patent threats.
I've seen this explanation in a few places and while I don't discount it entirely, I find it difficult to believe it's the sole factor at play; primarily because of the dearth of other third-party accessible services that build on AppleID (CalDAV and IMAP/SMTP are the only ones that come to mind).
I think it's probably time to move on from XMPP, but you really need Google, Apple, Microsoft and WhatsApp in a room together thrashing out something that'll work for everyone. I can dream, right?