Interesting that it has the same design aesthetic as the 6/6+ but has a smaller screen (4"). I bet a lot of people would want an iPhone the same size as this thing.
So I suppose that's maybe the key: whereas iPads and iPod touches are always meant to be held at arms length, iPhones are additionally expected to be held against your ear and are shaped slightly differently because of that additional use case.
Or maybe it'd be hard to make a "half-pipe" from aluminum on a device as thin as an iPod touch.
Or maybe it's all just arbitrary and everything in life is random and meaningless.
The recent evolutions of Apple's design often make me think that.