Is it true that part of Apple's success in consumer electronics is offering much less choice than rivals? Now, granted, you'd never hear someone saying "well, I was going to buy an Android phone, but there were just so many fewer options from Apple that I couldn't resist!" My point, though, is that choice is hard, and the near-infinite options you have for most items like phones or computers can be paralysing. When Apple offers you "phone" or "slightly bigger phone", suddenly you'd be much more at ease and more likely to buy. Couple that with post-purchase rationalisations about knowing it will "just work" and the fact that Apple designs shiny things, and everything lines up for them. Apple knows that you find choosing hard, and they design and sell based on that.
Mokalus of Borg
PS - Plus the limited form factor means that the accessories market is bigger.
PPS - It's subtle, but I think it's a valid strategy.