Voadam
Legend
I haven't checked but I would expect they are rarer on the random item generation charts as well being on one of the multiple 1e miscellaneous item subtables compared with the single rings table.My theory for the lack of cloaks of protection in modules is space. You could only wear one cloak, so too many protection cloaks would be wasted. People were going to want cloaks of the manta or cloaks of elven kind. Rings on the other hand were one per hand, so you could have an interesting ring and a protection ring. And everyone likes more defense.
Plus they were not in B/X like rings of protection +1 were so no Keep on the Borderlands early loot for a bunch.