Why would they? The helmet they use allows them to have shade in the desert heat and keep the sun out of their eyes. Their helmets being either leather or linen falls in line with what was cheap. Not like the Legion uses guns all that much anyway.
Lets also realize that the all metal WWII helmets were never that effective in the first place unless you bounce a shot. They were just there to give you an illusion of safety. Unless the shooter used a handgun with a 9mm-357 range, you are going to be in a shit load of trouble.
Anything more than a 9mm bounce will put you in a coma and likely give you severe brain damage. This is why Britain almost retired the technology in WWII before realizing something is better than nothing. If not puncture and kill you since you are using reclaimed metal from pots and pans.
In order to protect your head, they would need to design or replicate the combat armor helmet or a brand new helmet much like our current helmets. Not just simple metal bowls.
That costs a lot of money. NCR is rich, but it ain't that rich. Same reason they don't deploy true power armor beyond a few select instances.
The more complicated design also means more labor intensive manufacturing, which runs counter to the quantity doctrine the NCR follows, a WWII doctrine.
Unless it can be made cheaply, quickly, and in large numbers the NCR can't field it. This is the reason the NCR use the 5.56mm, as that round is easy to aim with.
If a simple trooper was fighting a legionaire experienced enough to have a hunting rifle, a marksman rifle, a Hecate, or the .44 they'd be dead regardless of head gear. Helmets cant protect against the rounds some of these things are sporting.
Lets go even further:
So $100 NCR = 40. 20 = 8. 5 = 2.
This comes up as $110 for a single helmet. Their gun costs comes out to about $1,420. Their armor is about $720.
So a single NCR soldier costs the NCR $2,250 counting a helmet and not counting salary. When you see how expensive this all gets, cost cutting makes sense. Especially since it lacks effectiveness at larger rounds and could never take a direct hit.