Well, here's a thing: a player is limited to 4 mounted images.
You'd think "Oh, that's probably enough", but you're ridiculously wrong.
Slot 0 is used by items.
Slot 1 is usually used by akimbo weapons.
Slot 3 is used for player effects.
So all we got is slot 2. This means we can only have one consistant image mounted to the player.
Therefore no armor + helmet combo.
Why not allow us more images to work with? Having one free slot is just not enough.