When devising a trauma care program for field medics, one of the draft revisions from years ago that I saw was that the NPA was preferred as it was easy to place, required minimal maintenence, and in field conditions intubation placement or maintenence was not practical.
As a disclaimer I was never a military medic, nor was I ever in combat, but just looking at the logistics of it:
You are in a gunfight, the scene is definately not safe
and therefore not exactly conducive to all kinds of invasive procedures.
You have to get to the casualty, there may be multiple.
You then you have to get him/you out of direct fire. (perhaps by yourself as "the best first aid is superior fire power" as I once heard from a SF medic.)
So if you use a King or ET tube, how are you going to bag this person/people?
It doesn't seem very wise to start pulling the fighting folk off the line to tend to wounded. At least not if you want to survive the day.
Triage for many years has advocated that positioning an airway, which is really what an NPA is designed for, is one of the most basic and useful interventions.
I am sure many of us can attest that even in the civillian world, ET tubes are a risk everytime a patient is moved. From direct fire, to a fall back position, to the evac, to the first doctor, to perhaps another area, seems like a lot of moves for tubes.
Perhpas some of the members here with actual experience would expand upon this a little?