It's a matter of signaling/conspicuous consumption. People with brand new iPhones have no way of distinguishing themselves from people with the old model.

It's also another way that Apple drives upgrades in the high end: by making the old model look old and dated.

And it's a pretty old model. I agree with you that's a beautiful piece of hardware, and I think it looks better than any of the other models, and better than any of the 'teardrop' mockups. But it's already been in the market for, what, 16 months? If they don't have another refresh until next year, that's a pretty long time to have one chassis.