You make an excellent point. I would guess that it is orders of magnitude more expensive for Apple to create a new hardware configuration than it is for them to add software feature flags, though. But, assuming the cost of making the hardware change worldwide exceeds the cost of reconfiguring their factories for new hardware, you're right that they would not choose to make the hardware change worldwide.

Almost certainly someone (or an entire team) carefully crunched the numbers and deliberately decided not to keep a Lightning US iPhone.