The full-screen mode handling might be a clue about what went wrong: if you swipe up from a space that contains a full screen app, it has an animation where the app goes into a slot in the preview strip, but that animation doesn't make sense visually for a non-full-screen space. So, perhaps someone was implementing that animation, didn't want to implement an alternate animation for the non-fullscreen case, and decided to minimize the preview strip instead? And because this was after Steve Jobs had died, there was no one left in charge of UX to explain why that was a bad idea?