Quality of steel was not sufficient enough to be practicable for body armor and helmets
Historically speaking, use of armor, shields and helmets declined with the advance of firearms. During the Napoleonic era, they were almost completely abandoned, except in heavy cavalry units that used them to protect themselves from cold weapons (swords, sabres, spears ...) not from firearms. Helmets and to a lesser extent body armor reappeared in WW1 and latter in WW2. Helmets did offer protection against shrapnel and glancing bullets, but body armor (like for example this Soviet body armor) was only issued to specialized units (combat engineers mostly) dodue to it being totoo cumbersome and unhandy .
Now, as you can see, body armor mentioned above protected only the breast, and only from submachine gun ammunition, yet it weighted 3.5 kg . Mind you, this happened in 1940's with already well developed steel technology. But in the 19th century, before the Bessemer process, steel was produced only sporadically with much lesser quality then after that. Contrary to that, sheer energy of muskets is comparable to modern firearms like the Ak-47 and the M-16 . Muskets are of course much less precise thenthan modern weapons, but to compensate for that they were usually fired en masse and from closer range. As a consequence of that, effective body armor would have to be at least 5-6 mm thick, with corresponding weight increase, to give protection for a relatively limited area.
As for helmets, note that at that time and place most of incoming fire was direct, unlike WW1 and latter. This includes artillery fire (canister shoot and cannonballs filled with gunpowder). Helmets usually protect only the forehead from direct fire, and the rest is dedicated to protection from overhead shrapnel and flying debris. Military caps from that areaera did offer some protection from direct fire, but they were mostly dedicated to stopping saber and sword cuts from above (like this bearskin capof Napoleonic Imperial Guard) . Shrapnel only became a major source of combat casualties with advent of static trench warfare in WW1 . At that time, the quality of steel improved sufficiently that both sides could issue various types of helmets relatively effective for stopping it .