These helms had allowed their user to view the entire map of the land, city, or dungeon level in an instant. The only disadvantage was that after a single use, the helm's power was used up. Since mapping helms did not afford any real protection to the wearer, they were typically discarded afterwards, as there was no way to recharge them.
Likely because of the great cost involved in creating large, one-shot items, these helms fell out of use soon after Minax's fall.
Essentially, mapping helms were the forerunners of peer gems, and were replaced by them in Ultima III.