A significant portion of Vita's third party support has come/is coming from titles being ported across from another PlayStation platform (or vice versa). Be it PSP (Toukiden/Gods Eater/Danganronpa) or PS3 (Warriors Orochi/Legend of Heroes/Dragon's Crown).
Of course, the situation is better for Vita since there's an exclusive here or there (Chronos Materia; Mind=0 etc.) but I wouldn't overplay the hand of third party support. There's clearly some system that makes porting between existing PlayStation platforms easy - perhaps Sony provides some software to do it.
There isn't such an easy situation for Wii-U. Can't port to or from Wii because it's dead. Can't port to or from DS because it's dead. That leaves 3DS; and I believe the power gulf between 3DS & Wii-U is miles bigger than that of PS3 & Vita (or indeed PSP & Vita?)
Of course, that begs the question of why developers aren't simply porting their PS360 versions across. And for that, I don't have an answer. Maybe where MS & Sony provide incentives to port, Nintendo don't. Maybe all the tinfoilhatting is correct and they're doing it just for the sake of being silly. Who knows.
But I think Sony for running 2 other "viable" platforms in Japan which provides a method for multi-platting games that would be a risk on Vita is the reason it has the third party support that it has. And the fact that Nintendo aren't providing this is probably a reason we're not seeing as much on Wii-U. There's probably other reasons too, ofc.