I think it was a philosophy more than anything. Fullscreen is not just "taking up the whole screen", it's the idea that you are working on only one application

I think it was a philosophy more than anything. Fullscreen is not just "taking up the whole screen", it's the idea that you are working on only one application. So if you are fullscreen on one screen, the other screens should be blanked out so they aren't a distraction. Many disagreed with this philosophy, but that was why it was "broken."