It was never about ease of development on the Wii U. It's simple business 101.
Ever since Sony and then MS came into the picture, third-party sales have tanked on Nintendo "consoles." Why develop a game for a Nintendo console when chances are you won't even make your investment back?
Handheld market is a different story seeing as Nintendo has no competition there. Vita? please, Sony needs to do a better job there if they want to compete with Nintendo. Third-party will sell on the 3DS because there isn't an alternative at the moment unlike the console market.
People like to talk about the 3DS/Wii U mutually but their markets dynamic is very different. What works for the 3DS will not work for the Wii U due to competition.