I would add a little bit to the "primers popping out", it wasn't concern over blown primers "popping out" (which, while rare, can happen) but concern over the primers of (primarily rifle) ammunition used by machine guns, particularly belt fed.
The ammo in a belt is subjected to a lot of vibration as the belt is fed through the gun, and while I don't know of any specific instances, it's quite likely some where, some time, rounds were found with primers that had been shaken loose or out.
Crimping cures this. And I think its quite likely that the military just adopted a single standard (primers will be crimped) for all small arms ammo production or purchases.
Ammo made with the expectation of military sales will meet the required specs for crimp and sealant. Ammo made for everyone may not.