The Fourth Wall

The 4th wall is the thing that separates the fiction from the non-fiction. Said wall originally didn't exist at all, but at some point between Nightmare Fredbear being revealed and the end of FNAF World's Update 2, it was added (likely around the time the first crossovers showed up).

Once other fictions joined forces, the crossovers relied on the wall. So when it was broken, things broke.