Because of Multitype trick wont work. If it did arcues could change type mid-battle which would be too overpowered, being able to change its type to take on teams better.
>Multitype changes the type of the Pokémon with this Ability depending on the Plate attached. The item-removing effects of Thief, Covet, Trick, Switcheroo, and Knock Off will not work on a Pokémon with Multitype.-Source