Combat helmets were reinstated through application of military science.

In the Napoleonic era, the main creator of casualties were direct fire weapons, essentially things that you would point directly at a target and shoot. Napoleonic field guns were essentially direct fire weapons as well, firing solid shots at a low angle.

Indirect fire weapons are weapons that are launched at a trajectory, launched into the air and dropping down on their targets. Prior to WWI, such weapons are used in sieges to drop stuff over walls.

Solid projectiles in the 19th century like musket balls and cannonballs were too powerful to stop economically with armor. Even today, combat helmets couldn't save you from a direct hit from an artillery shell, or even a modestly powerful bullet.

However, Combat helmets do protect from falling debris and low velocity metal. In WWI, entrenched infantry were mostly safe from direct fire (aside from the unlucky direct shell), but they could easily be hit by metal and torn-up ground debris hitting them from above. After helmets were introduced, deaths due to head trauma plummeted (Injuries due to head trauma concurrently went up, up, up).

But artillery was still incredibly powerful. Even in WWII artillery accounted for something like 70% of casualties.

So how did nobody plan accordingly? One reason is that there were 40 years between WWI and the Franco-Prussian War, the last big war between European states. Artillery was a big killer during that war, but it wasn't the main killer. Furthermore, those artillery guns were still mainly direct fire weapons, just with exploding shells that were more deadly that solid cannonballs.

In those 40 years, targeting systems became reliable and widespread, gun ranges got longer and longer, and explosive power got higher. Machine guns are what we see when we imagine horrific industrial warfare, but it really should be artillery.

This holds true even today, albeit with rockets and guided munitions to supplement dumb shells.

Weapons and armor had been in a sort of 'arms race' throughout human history. As weapons adapted, so too would armor. We can see the phases armor went through in ancient and medieval Europe to combat the horrific ways we discovered how to kill someone. Helmets were obviously invented to prevent head injury, primarily against archers.

Warfare in Europe was largely melee. There was actually somewhat of a stigma against ranged fighting that can be traced back to Ancient Greece. At the time, the virtue 'arete,' meaning excellence of any kind, dominated the battlefield. Attestations of this can be found in The Illiad, in fact. Excellence in combat, aristeia, could be a little rigid. There had to be equal risk for both sides or it wasn't an 'honorable' fight. This value waned for a time, but one can still note its presence until the mid-2nd millennium, when warfare changed forever.

The invention of gunpowder (notably rifling) meant that melee combat was growing rarer and rarer. Once guns became common in militaries, armor went from 'the thing protecting you from arrows and swords' to 'the heavy thing that did nothing to stop a musket ball.' Armor was useless at the time and battlefield tactics of the time required men to be fast and mobile, not tough.

In WWI, helmets made a comeback because warfare was so different from the Napoleonic era. Guns had changed, bullets had changed, and tactics had changed. You didn't have people lining up in combat anymore. People were fighting in trenches. If you're fighting an enemy in another trench, your head remains very vulnerable. Worse, anyone outside the trench was above you and could very easily stab you in the head. Trench warfare was absolutely horrible.

Helmets save lives against artillery BIG TIME. Obviously, if you get hit with a shell, you're dead, but that is relatively rare. The danger is shrapnel. The metal parts of the shell, rocks, debris, and all kinds of things that you don't want inside your head are what make artillery shells dangerous. Even today, we rely on things that throw shrapnel everywhere at high speeds to take out lots of people at once, which is why the helmet is still used today.

I hope this answers your question!