Put yourself in the position of a very good software engineer. You're making games because you're in it for the love and not so much the money as you could probably do uninteresting non gaming development and earn at least twice the salary with better hours. Publishers and managers only have so much sway on you because the chances of being unemployed for more than a couple of weeks between jobs is pretty slim. This is the challenge of managing a talented employee base which has mobile skills and a real willingness to walk if they don't get what they want.
You have two major options, either you maintain old code and port the Wii U or you can work on completely new architectures and take on new challenges so which one do you pick? Remembering that you're in it for the love and not the money so unless you have a specific love for Nintendo you're probably going to pick the latter. Now multiply this effect by X number of engineers spread over Y number of development houses and you have the answer as to why Z numbers multiplatform ports aren't coming Nintendo's way.
So there we have it, a repeat of the last generation where 3rd party developers pretty much only develop for the Nintendo console because they 'have to' and any developer with a choice exercises it generally and develops for other systems. Nintendo was simply unwilling to do what the top level developers wanted and hence they don't get the support from the top level developers, had the Wii U been more powerful it would have been a lot more interesting to developers or had it come say in 2010 there wouldn't have been the option for these developers to simply move onto new systems.
TLDR: Top level developers would have no problem supporting the Wii U but they just don't want to and managers can't tell them what to do.