score:13
The Medieval helmet was designed exclusively to defend against hand-held weapons like sword and lance. With the arrival of the musket, all infantry armor was deemed superfluous, as was most cavalry armor. For the cost of material, the extra weight of carrying it around, and the additional exhaustion from wearing metal in the hot summer campaign seasons, no benefit was seen even for senior officers.
The turn-around came with the arrival of the breech-loading rifle, the machine gun, and the compensating defenses of the fox-hole and slit-trench. Whereas a helmet-equipped infantry man of the Napoleonic period is only protected over 5% of his exposed target area by a helmet, an entrenched infantryman of WWI achieves 90% protection from his helmet. This vastly increased effectiveness accounts for the helmets comeback.
Update:
Whether from bullet or shrapnel, a helmet alone provides little cover (protective or not) for a soldier standing in the middle of an open field, while providing significant cover for a man prone, entrenched, or behind hard cover.
Upvote:5
The main reason for helmlet return was protection from artillery shrapnel. WWI and WWII helmlets cannot protect from bullets, entirely. But they well protect from shrapnel from shelling which greatly minimizes the casualties