I would like to bring up the point of high-velocity projectiles and explosions taking the vast majority of offensive weapons on today's battlefield.
Ian would probably have much more information than I do, as he serves, and is very well-versed on such things.
The impacts the human body is being subjected to in war are at incredible levels of force. One does not even need to be struck by a round or be in immediate proximity to an explosion for it to exert much forse upon one's body. It doesn't take that much to mess with the brain, and these forces can easily mash the thing up against the skull. (for comparison, merely touching it breifly is what gives one a concussion)
I would wonder if this would also play a factor. Remember, our ancestors were exposed to forces much more mundane compared to what we regularly subject our enemies to today.