I don't think 3rd party devs fail Nintendo. I think they try, fail and give up.
I'm part of the problem. For my Nintendo systems I buy mostly Nintendo 1st games, mainly Mario, Zelda and Metroid. I buy a few3rd party exclusives, but for me gaming systems by Nintendo are Nintendo game boxes. When I buy a Wii U it will be for Mario, Zelda and/or Metroid just like how I buy Xbox for Halo and Playstation for Team Ico games.
Like I said I'm part of the problem, but it's the millions like me that have convinced 3rd parties to risk less on Nintendo consoles.
I'm sure 3rd parties looked at the Wii sales and giant userbase and thought it would be the land of milk and honey, but instead they got lost in the desert. They watched their games die on Nintendo machines while Nintendo sold their games at record numbers for years and years. Mario Kart Wii continues to sell well and has barely dropped from it's launch price. 3rd party games don't sell and then they go to bargain bins and don't sell.
I'm not really sure how Nintendo can fix this. Their fans, me included, love their games. They make plenty of them and that's what we want. Nintendo fans get what they are craving for direction from Nintendo. I guess 3rd parties need to convince non-Nintendo fans to buy Wii U for their games instead of Nintendo games. But so long as Nintendo games dominate all that's left is crumbs for others. I can't really blame 3rd parties for not investing tons of cash and their best talent while the risk is so high.
