While it's easy to find reasons as to why most 3rd party games sold poorly on Wii U (as explained by aielyn above), but if this trend continues it will spell trouble for Nintendo later on.
Personally, I want more 3rd party games than 1st party right now - but that's because I haven't bothered getting a PS360 or getting the games for PC...
I'm on Twitter @DanneSandin!
Furthermore, I think VGChartz should add a "Like"-button.