Was wondering why cloaks can't be worn with armor. They aren't robes or any other type of armor. They are an accessory similar to gauntlets, rings and necklaces. They are supposed to add to your AC or bestow a magical affect. In Beholder they are treated as armor. Why is that?

Because the game's ancient.

I agree. In might and magic 3-4-5 they work exactly like gauntlets, rings and necklaces, along with belts.

Might and Magic does not use the Dungeons and Dragons rules or settings, though.

To the OP, like Gotcha said, it's because the developers were trying to port a 30+ year-old rpg ruleset into digital format, and altered some rules to fit what they thought was better game balance.