Because the Apple ][ was a locked design, folks started poking directly into ROM and RAM locations, for instance directly manipulating system variables, or even jumping into the middle of ROM subroutines. A book called "What's Where in the Apple II" documented almost every known hook.
My mom had that book. She's my hacker role model.
As a result of the way that software interacted, it became virtually impossible for Apple to update the ROMs without breaking popular apps.
The next generation of personal computers all represented different approaches to avoiding this problem by providing well documented entry points while offering no guarantee of long term code stability. IBM made one mistake: They let people hard code the address of video RAM, which is what led to the legendary 640k barrier.