If you lowered your hood / hid your helmet you'd start complaining with "my head has nothing on it, why can't I wear a helmet!?"

Put simply, the multi-piece armours are one model. In order to allow a /hood command we would need separate item models for the hood and the robe, thus removing the whole point of these items in the first place.

This issue came up in beta when people wanted a /hood command for the WHM's AF robes to allow them to raise their hood.

While it would be nice, don't expect it on head+body armour pieces.