Because at the time, advances in firearms were ahead of advances in armor. Not until 1915 was the technology there to make a helmet that would stop a bullet.
This is actually a misconception about WWI-era helmets. The English Brodie helmet couldn’t really stop a direct round from a machine gun. What they were best suited for was protecting from falling debris and shrapnel that exploded overhead, and they could handle ricochets. The curved shape of the Brodie could at times deflect bullets, but they couldn’t stop them.
But Professor Attewell is definitely right, the advances of rifling technology in the US Civil War far outstripped their defensive capabilities. The new Minie ball which had debuted in the Crimean War made rifles far faster to reload, and the new shape of the bullet meant that instead of flattening, the bullet would corkscrew into its victims, splintering bone and causing massive compound fractures. This is why so many WIA’s in both the Crimean War and the US Civil War required amputations compared to eras past.
-SLAL
I thought the stahlhelm could stop a rifle bullet, no?