The design of the Brodie helmet (or M1917 for the American-made version) is really a product of the conflict that it was introduced during: the First World War. The executive summary here is that of all the things that can kill you in trench warfare that a helmet will protect against (so excluding chemical attacks, actually machine gun fire while attacking an enemy trench, that sort of thing), someone trying to bombard your dugout with artillery is the one you're likely to face most often. By comparison, the likelihood of someone throwing a grenade into your trench is low on a day-to-day basis, as is the risk of stepping on a mine.

In conflicts after WWI, infantry regained some of their former utility as maneuver elements, which meant they were exposed to all the usual stuff (grenades, mines, that kind of thing) again more regularly. Beyond that, starting with WWII there existed an increased emphasis on urban warfare, and after WWII there was a dramatically decreased emphasis on symmetrical warfare. And in general in asymmetrical warfare if you're on the side that issues purpose-made helmets, you're also on the side that's using the artillery.

TL;DR On a modern battlefield a regular army soldier can expect to encounter a wider variety of explosives, and most of them are being used by people without access to airburst artillery.

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