The reason I believe it's not coming out for the Wii U was because the original Luigi's Mansion was to be in 3D. Yup, that means ever gamecube game you had/ or wanted to have was to be in 3D too, why? Because the gamecube was to be a 3DS but for a bigger console. However the plans failed and therefore only made it 2D. It wasn't until after the 3DS where they could make up for what they missed for the old one as a "make up" kind of act. The reason why it's not actually coming is because it would ruin the entire purpose of the meaning of it being only 3D.