How do you know? It's was unprefixed already ~3 months ago, so why should it happen with the next release? And when does the next release come? And why do their release cycles are not as short as those from Chrome or Firefox?
Because it's not their core business.
> ... to be lack of contributors on WebKit's part than anything.
Exactly what I wrote: Apple is focussing on other stuff than the web because—again—it's not their core business. They have two browsers, mobile Safari and desktop Safari and none of them got this important upgrade yet. A fast innovating web isn't in Apple's favor.