Apple pride themselves on making the iPhone simple, and not fragmented. I can't see how they can do that and still release the iPhones in different styles, unless we're talking nothing more than different covers.
The massive market growth in smartphones is being caused by the smartphone becoming cheaper - if they want more of the market then they have to enter that area.
Very good question. For some reason having differentiation in the smartphone space is known as "Fragmentation" and bad.
Personally, I'd be very happy with Apple bringing out a wider mix of phones, but they seem dead against it.
Edit: Just seen your comment at the bottom of the page - I totally agree - Apple's lack of different options is probably going to go badly for them in the long run.
To make development easy and to make it easy create good user experiences across all iOS devices. None of their iOS devices have multiple versions in a single product cycle.
The massive market growth in smartphones is being caused by the smartphone becoming cheaper - if they want more of the market then they have to enter that area.