The only reason 3rd party software hasn't lit up the charts on Wii U, is because the console itself has sold slowly in the opening months. That's it, that's all. If you look at trends with the Wii, with the Gamecube, even the N64, it's about on par. Meaning Nintendo's game since N64 have always sold the most, the highest selling going many millions. There are 3rd party games on each of their consoles since that time that have been huge hits. But in general, even the hugest hits on Nintendo consoles still don't equal the sales of the hugest hits by Nintendo themselves. And the thing people are ignoring is that this isn't even unique to Nintendo. It was the same way with Sega. On all of their consoles, Sega-made or Sega-published games were almost always the highest sellers, even compared to big 3rd party hits on their systems. The problem is, even though Sony and MS have their "own franchises", it's a bit of a different beast. With the exception of perhaps Halo, their first party games don't often reach the same heights as big Nintendo hits do. Big third party games, such as Call of Duty, etc., tend to be the biggest sellers on Xbox and PS nowadays. And even going back to PS1, in general, third party hits were the biggest selling attractions. Like I said, it's just been a different beast.
When it came/comes to Sega and Nintendo platforms, while 3rd parties have and will continue to be successful (yes, we all know Sega's out of it now, I'm just comparing), people always wanted/want their First Party franchises the most. That's just how it's been, historically. I'm sure if you looked back, it was probably mostly the same way for Atari consoles, and I know that it the case for the Turbo Graphx 16 and Hudson Soft's games.
But regardless, circling back around to the original statement, the main reason third party games haven't sold huge on Wii U yet, is because A) many of them were ports, B) many of them have sold decently in digital downloads which aren't tracked nearly as well, and C) the Wii U itself hasn't picked up steam yet. Looking at Zombi U, you can see that it's sales, while not great, if you look at it from an attach-rate standpoint, as I said, is more on par with past Nintendo consoles. I think if the Wii U had sold a lot more consoles right off the bat, Zombi U would likely be a million seller right now, based on that. And I think it's pretty obvious games like Monster Hunter 3 Ultimate and Lego City Undercover are already shaping up to be substantial hits, that will help drive system sales, which will in turn help sell some of those other third party games.