If you read stories about the iPhone's development, you come across anecdotes that Apple tapped its top engineers to get the iPhone out the door. They worked long hours and weekends. Many lost spouses and relationships. It was extremely taxing. Again this is another anecdote, but it seems many of these engineers left around the release of the iPhone 4. The iPhone pipeline became a never-ending treadmill.
The tax of losing much of the top talent across all engineering functions has slowly taken its toll on Apple since. I would not be surprised if the institutional knowledge of OSX has either left or been promoted up the ladder, leaving that team with new hires unfamiliar with its inner workings.