There is no logical reason that a competing handheld console should be outselling a home console like the Wii U. The only handheld console that should be outselling the Wii U is another Nintendo console like the 3DS. It just doesn't make any sense.
It's only barely outselling it in Japan, the goddamn HANDHELD country, and even then the Wii U is still overall doing better. The Vita's getting an increase in sales as its actually getting SOME games.