I've thought about this a bit, and what I can come up with is that Apple had the clear lead in the first wave of home PCs in the late 70s (the others being the Commodore PET and the TRS-80 model I), and maintained it. The Apple II had bitmap graphics and colour built-in, and a very fast and relatively cheap disk add-on, but also well thought out expandability. You didn't need to buy a sidecar unit; just throw a card in an empty slot. Importantly, it also worked with inexpensive TVs and monochrome monitors that you could purchase separately. The hardware was also high quality - it had a nice keyboard, and a switching power supply that didn't get hot.

Fast forward a few years, and the Apple II was still very usable and competitive, with RAM expansion options up to 128k, higher res graphics, and 80 column text, while still supporting the same software.

One other thing is that the Apple II was wildly profitable. It had no custom chips, just cleverly used commodity chips and some ROMs. This includes the fast and cheap disk system.