I think the real answer is "well we already started working on this game and we figure the PS4 and Xbox 720 will come out soon and we're going to move to them and we figure the Wii U will not be powerful enough to handle multiplatform games with those systems so why bother? The Wii U will be 'on par' for about a year and we can't get anything out by then anyway." They also probably figure this will become the casual system like the Wii and that the audience for their games won't be on the system anyway.
Nintendo worked REALLY hard at pissing off third parties during the NES and SNES days. The N64 cartridges was the last straw so they lost almost everybody overnight and those third parties were probably GLAD to be rid of Nintendo. And since then what the hell has Nintendo done to win them back? The Cube was the most third party friendly system they've released since then and it's appeal was pretty much that it was not grossly incompatable for ports like the N64 and Wii were. Hooray, Nintendo adequate! I so want to work with them now. Everything I've heard about Nintendo's relationships with any outside company is that they're hard to work with, treat everyone like competition, and always try to gain some advantage. This isn't just with third parties but also with retailers and the gaming media. It's not just that Nintendo doesn't offer moneyhats, they're just not easy to work with and provide no incentive for anyone that wrote them off to consider coming back.
On the N64 they had secret microcode so that their own games would have an advantage over third party titles. On the Cube if you asked them how to go online they said "I don't know. You figure it out" and so no one did. If you've ever worked with someone who is a huge pain in the ass, you don't work with them again unless you have to or the opportunity presented from working with them is so great you can't ignore it. So do these companies NEED to support the Wii U? Is the Wii U such a gold mine for third parties that they would be complete fools to ignore it? If they can skip the Wii U and still be successful, then why not do that?
Even from a creative perspective who cares about the Gamepad? Who has had this awesome idea in their heads for years that only now the Wii U can allow for? Nintendo acts like this is a big deal but it's a forced gimmick and no one, not even Nintendo devs, have been waiting for a system with this functionality. Is it worthwhile to deal with Nintendo if you don't need them to stay in business, it won't bring about any major financial gain, and you don't require the unique features of their system to bring your idea to life?
Another thing to remember is that while publishers are businessmen, developers are creative people and the better ones that have the clout to do so, want to make the games they themselves want to play. Nintendo's whole Wii approach was not inviting to creative developers. Who the hell dreams of making casual shovelware for rubes? No one, so any dev that isn't just in it for money or isn't some small time team that just gets stuck making games for Pixar movies, is not going to want to be part of Nintendo's strategy. The "core vs. casual" backlash that the Wii created exists with developers as well because devs are typically gamers. They don't want to make or play casual crap and they don't want the industry to turn into casual crap so they have no reason to specifically help Nintendo be successful unless they have to.