1) Apple still develops the OS using waterfall over the year. Entire sweeping changes are made only at x.0 releases that trickle down to teams that have to work around the instability all year long and there's no other approved way to get in significant changes.

2) They keep adding more apps to the core OS image that can only be updated with a full software update now. This makes delivery of quick fix updates near impossible since they have to go through the OS release management teams.

It certainly sells better to have a huge list of changes at WWDC that then become reasons to upgrade, but software delivery has moved on from waterfall, so in that respect Apple's OS teams are behind.