I think they simply made their research and found some of their test form factors better than others for the targeted public (or simply liked them more), so they based more devices on a similar form factor/design while including different hardware/software to differentiate the models...
This probably gives lesser doubts/confusion in the buyers, while wasting extra time/money on unneeded design research.
And still, I find Nokia to be one of the few to have a big design variation in their different models and to offer a wide range of products so I can pick the one that fits me best...
On the contrary, Apple focuses on one single phone (like the form factor/hw/sw or not.. or let yourself convince by SJ's "amazing", "awesome", "great"...), and other brands on just a few ones.
The only field they lack presence at the moment is the high end one (apart the N900), which is coming hopefully.

And remember that when you build a touchscreen phone you don't have so many options in design... it has to be a parallelepiped with a big screen and be as thin as possible.. you can round its angles a bit but then?
And optional qwerty kb, for which the hinge is a great design factor imo.