when I put my Pokemon with an item into the pokebank the item disappears
wy can't I take the items with them
The Pokebank is programmed to send any items of any Pokemon back to your bag in the game that you are using Pokebank with. This is possibly to ensure a Pokemon with an item from ORAS, for example, doesn't go into a copy of X and Y with that item and corrupt the game. Because that item is not anywhere within the coding of X or Y, the game will be unsure of how to deal with the item, leading to a corrupted save. This also applies to Pokemon from BW or B2W2 being transported to XY or ORAS with an item that doesn't exist in Generation 6.
Hope this helped.