You can't place armor on armor stands if the item is in your off-hand.

Steps to reproduce:
(I have just tried it in Survival, I don't know if this doesn't work in creative either)
1.- Place an armor stand.
2.- Put any kind of armor item in your off-hand. (Like a leather helmet or so)
3.- Equip yourself with full armor, so you don't equip yourself with your off-hand item.
4.- Try to equip your armor stand by right clicking it with the armor item in your off-hand.
The armor stand doesn't get equiped by your right-clicking action.

I think this might happen because the game thinks you are trying to equip it on yourself first, so it fails to do both.