It's not similar to height distribution. At the population statistics level, human height is bimodal because humans either have a Y chromosome or they don't. What is the equivalent binary variable for the phone market?
I suspect there are (at least) two populations of phone buyers: those who see it as a cool gadget with the latest technology, and those who just want a device to browse the web, take some pictures, and play some games. The former upgrade every year or two to the best phone they can afford; the latter are happy paying a few hundred bucks or less when their old phone breaks or no longer holds a charge.
The interesting part about IPhones in particular is that they are status symbols. I've seen plenty of working class folks, especially women, flaunting IPhones.
I'm sure part of the reason is that since most carriers (and Apple itself) will finance the phone cheaply, so the practical difference between the cheapest and most expensive phone is $30 a month.
