The same could be said of the Stormtroopers, perhaps it was just easier and cheaper to make their armor than those of the clones even at the expense of protection, especially when you figure that even Clone Trooper armor couldn't stop blaster fire so perhaps the Imperial military figured that cheaper was better since the extra money didn't result in extra protection.
You can actually see that at work between the two. In the couple years of the clone army's existence we've seen, we have no less than a dozen distinct helmet types (plus a couple equipment variations within some of those), most presumably for a particular mission role or advantage over another. Maybe a third are evolutions from one to another, but that's still really rapid evolution. Meanwhile, in the OT, we have four helmet types all throughout. And two of those -- infantry and pilots/drivers -- share a common base (the faceplate) and thus, presumably, shared parts for ease and lack of cost for repair and maintenance.
--Jonah