The Wii U generation is more clearly multi-console owners, and they prefer to buy the 3rd party software on their other system, because of better online services on system level (party chat, easy access to friends lists, etc) and/or as simply due to lack of achievements/trophies on the Wii U.
The gamers are choosing the options available in the marketplace that make most sense to them. So in short all blame lies with Nintendo, for not understanding and not catering to the needs/wants of those gamers, especially when they have the option to get those things on other systems.