Budget cuts, delaying releases for arbitrary reasons, more and more technical debt on key portions of iOS seem to be the norm rather than the exception. Each subsequent release of iOS is more bloated than ever, and teams are reluctant to port their stuff over to Swift because of the aforementioned technical debt.
Fundamentally, the problem is that the software org at Apple is a third class citizen. In order for that to change, there needs to be a shift in resources toward the SW org. But I don't see it happening any time soon :)