This design is completely modular, with the working parts mounted in a steel carriage that can be swapped into any number of different-sized grip frames. This crosses over to the civilian/commercial market - if a grip that is tailored for smaller female hands is offered for the p320 series, a military version will be available too. I can see why the armorers would like that design.
Currently, I don't think there are any American designs that offer this sort of flexibility.
It (this modular P320 design) is a neat idea. But I have seen at least one P320 that could not hold a group at 5 yards. The receiver group, slide, and barrel would ‘rock’ in the plastic frame on firing (unlike a conventional design where the receiver and frame are one), thus a different alignment every shot. Others seemed fine, but seems like a design issue to me. When they actually get through all the recalls and any ‘new’ issues, I might get one.
The question is not “why SIG” but why the Navy needs 60,000 new pistols.
Tokarevs had it decades ago ...