In an interview with NowGamer, chief technical officer Oles Shishkovtsov went so far as to say that the "Wii U has a horrible, slow CPU.” THQ's Huw Beynon, echoed Shishkovtsov's sentiment, saying that all the effort involved in porting Metro: Last Light simply isn't worth it.
We had an early look at it, we thought we could probably do it, but in terms of the impact we would make on the overall quality of the game – potentially to its detriment – we just figured it wasn’t worth pursuing at this time. It’s something we might return to. I really couldn’t make any promises, though.
This certainly doesn't bode well for the future of third-party support on the Wii U. We'll just have to wait and see whether or not Nintendo will be able to convince other studios to develop for their console in the coming months.