Reasons? Because Wii U has the innovative gemupaddo-desu, and because they'd feel like making one. There, good enough?

beeing the only big 3rd party isnt that bad.

I cannot. I'm a Nintendo fan and the last few years have proven to me that Nintendo gamers don't care about third party games at all.

It doesn't bothers me because most 3rd party games are seriously flawed - flaws that never would be excused on a 1st party game from nintendo but somehow gets overlooked from reviewers - so I don't need that kind of bugged efforts.

That, and Nintendo always makes the best games of every generation and since I have to work most of the day I don't need that many games anyways.

kinda old thread but THIS.
It makes much more sense (and money) to let a group of your own people work on your next big project and release it therefore earlier, than to invest them into a project that generates no or only few amounts of money. Even if it takes several games, but there will be a point where you can release one additional multi million seller that will easily make up for all other ports that could have been.

1. Look out for a new market which IS actually on Wii U too. Even if Rayman Legends didn't sell gangbusters, it sold better on WiiU than on any other console. Build upon an established demographic and gain respect of Nintendo console owners.

2. Gaming development is also about building upon exciting new features and find new ways to play and enjoy games. Asymmetric gameplay is one of them features.

Also, 3DS proved (at least, in Japan) that 3rd party games can sell (Puzzle and Dragons, Monster Hunter 4, Dragon Quest VII)