Looks like TT Games can only handle one LEGO-related Wii U game at a time. Here's what a TT Games rep said when asked why LEGO Lord of the Rings isn't on Wii U.

"The primary reason is that [Traveller's Tales] and Lego were already working on Lego City Undercover for Wii U."

I'm actually okay with this. I need a breath of fresh air with the LEGO games, which LEGO City Undercover seems to be. If you really want your LEGO Lord of the Rings experience, you always have Wii, 3DS and DS.