Why were helmets not used in the Napoleonic wars?

It's interesting that it took until WW1 for most armies in Europe to have helmets as standard issue from the time they fell out of use in the early modern era.
 
During sieges Napoleon's sappers used steel helmets and cuirasses. So did heavy cavalry at the time, although their helmets served mostly as protection against saber. That, and they also made soldiers look good.
Why they did fell out of the favour for a line troops? Well, for a long battles were generally decided in open field and by musket/rife fire or cannon fire. Cannon fire is more direct, than, say, howitzer fire, so shrapnels attack more horizotally than vertically. That made helmets useless, since they offered no protections against such fire. Of course there were sieges of American Civil War or the Crimean War, when indirect fire was more common, but I do not know wheter helmets were used at the time.
WW1 meant come back for helmets, because they served mostly to protect soldiers from shapnels and debris raining down on them while they were in trenches. They did not protect form direct fire, most rifle bullets could easily pierce a WW1 helmet, unless from very large distance. Notice, that AFAIK no army used steel helmets in first years of WW1, before beginning of the trench war.
 
Well the steel helmets in world war one were made in response to shrapnel wounds to the head as troops engaged in trench warfare, they were not meant to stop bullets (in many cases they couldn't) in the Napoleonic wars helmets were worn (see below), mostly by heavy cavalrymen, the same guys who were also the only ones wearing body armor extensively which was more for protecting against sword blows.

Shrapnel to the head wasn't as much of a threat to troops in the 1800s, not to mention no helmet is going to stop the ever so popular round shot from cannons.

1812.jpg
 
Is the Bearskin worn by British Guardsmen a helmet or a simple hat? The latter would seem the obvious choice, but I imagine it does offer a limited protection due to its thickness and cavity inside between top of hat and top of wearer's head.
 
Shakos and other tall hats like bearskins and mitres offered a respectable level of protection against sword blows.
 
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