Borderlands 2 isn't showing up on the Wii U because Gearbox couldn't think of any features which compelled them to take advantage of its special capabilities. Or, in the words of CEO Randy Pitchford, no port ideas made them say "OMG."
“We get asked if there is going to be a Wii U version of Borderlands, and the reason why there’s not is because we couldn’t think of a natural, obvious, ‘OMG, I want that for what the Wii U brings to the table’ feature,” Pitchford told GamesMaster magazine (via Nintendo Everything).
Borderlands 2 executive producer told Brian Martel (via Eurogamer) in July that the company was taking a wait-and-see approach with the console, and it would probably do inventory management with the second screen, if anything. However, he said Gearbox is excited to release Aliens: Colonial Marines on Wii U next year, a sentiment Pitchford shared.
“But with Aliens, the first thing we thought was ‘Holy sh*t, I can have a motion controller [we assume he means tracker] in my hands!’ When it’s there in my lap it feels like I’m part of the movie.”