Helmets have always been kind of an oddball item in classic D&D. As the numbers for Armor Class are fairly large, there isn't much room for fine division. The usual categories are: unarmored (AC 9 in D&D, AC 10 in AD&D), shield only, leather, leather & shield, mail, mail & shield, plate, plate & shield. AD&D's additional armor categories complicate this a bit, but I've always found the number of choices in the core rules - at least, in 2nd Edition - to be a bit excessive; most of my players stick to the ones they know. I have a feeling that the addition of studded leather, etc. was done to give thieves a slightly more advantageous option than simple leather armor, especially considering their fairly low hit points.
But helmets? Well, they're listed in the armor section of the AD&D2 Player's Handbook, with a price, but they don't seem to provide any benefit in the core rules themselves. I know that The Complete Fighter's Handbook has rules for piecemeal armor (including factoring in helmets); since many people consider 2nd Edition to be "king of splatbooks", it's possible that the designers left helmets there to make for a smoother addition with the PHBR series (the reddish-brown books). One of my players found a bronze helmet in a small treasure hoard, and wears it mostly because she likes it, even though it provides little to no benefit to AC.
There are a few notes, however, that suggest helmets as an assumed part of the metal armors; the notes for rogue skills in the PHB say that the thief must remove their gloves to Pick Pockets or Open Locks, and must remove their helmet to Hear Noise. It might just be one of the tendencies toward granular, literal-minded rules carried over from 1st Edition. After all, characters are expected to choose and spend money on their clothes, shoes, and sheaths for their weapons!