Yerm on 01 May 2018
it is entirely possible that this was an error. my own personal guess is that since the Switch is very young and both it and the Wii U use the eShop, they likely run on the same servers. so my theory is when the Switch version was released on the eShop, it updated the entire listing for the game to Switch only, meaning it would not show up on the Wii U eShop.
