There are many reasons, but one of the big subtleties that should get remembered is that the Apple II had essentially two great epics:

Epic 1: The Apple II sold with no expansion cards, but many expansion slots. Hackers and business designed addons for years.

Epic 2: the Apple IIe (and later IIc) were sold with an optimal set of expansion cards.

So you had one generation of experimentation and a second generation that leveraged all the hard work!