This may sound like a silly question, but here's my reasoning for it: Each day that goes by, I see more and more third party Vita titles announced for localization, while countless 3DS titles from third parties, some from very big franchises, remain in Japan with no signs of a localization anytime soon.
Let's take this back a bit, back to the Wii. That system saw a great deal of third party support, but the VAST majority of the support provided was in the form of shovelware, with the rest being mostly niche titles. Wii saw tons of "test games" and "experimental games", but third parties refused to provide a decent amount of true support in the form of AAA games. There were exceptions, sure, such as Monster Hunter 3 and Tales of Graces, but for the most part, this seemed to be the case.
It seems basically just like a different interpretation of history.
Many devs and publishers were hesitant to just throw all their might behind Wii after what happened with N64 and Gamecube, to say nothing about their history with Nintendo when they were at the head of the industry (extremely negative, to say the least). That's not about 'hating' them, though, it's a simple business decision based on historical precedent, which is about as reasonable a thing as any business should do.
Similarly, when they did put games on the platform - even high quality efforts like Zack & Wiki - they generally performed under par. There were plenty of games for Wii, but because of its unique nature, unique engines and unique gameplay concepts had to be developed for it, and because many devs invested on a multiplatform future - PS3/360/PC - Wii remained the odd one out, because it cost more to put real effort into it and you only really had ONE place to recoup those costs.
These again are not about hate, it's about making intelligent business decisions.
Even the DS had a tough time early on, despite being the successor to the GBA, as third parties quickly backed the PSP and pushed hard for that system to succeed. That followed on to Vita, as I stated earlier, with Vita getting a lot of third party support, and third parties being all-too happy to bring over Vita content as opposed to 3DS content, despite Vita selling well under even Wii U levels every month in the West, and under-performing even in Japan, the land of the handhelds.
DS had a tough time early on because most people - including Nintendo for an extremely limited time - seemed to be imagining it as some third pillar as they went to create their true GBA successor. Even they were unsure that the dual screen concept would take off (look at what they were using to demonstrate it early on: Mario 64!), and because of that the early DS days were sort of half-baked; a shit system design that was bulky and always a little uncomfortable, and a wasteland early lineup that included such "gems" as FEEL THE MAGIC XX/XY and what is possibly one of the worst Ridge Racer games ever made.
When they got a redesign and the PSP didn't quite take off the way people expected in that first year, Nintendo released a redesign and several games that changed perceptions - Nintendogs, New! Super Mario Bros., Brain Age, etc. This, in company with the redesign, colors and a new approach that emphasized most its unique utility versus the PSP, things shifted. People saw that the DS was the true GBA successor, that its unique functionality gave it a critical edge the public wanted that the PSP could never match, and businesses moved accordingly.
Again, not hate to my ears...
The Wii U is currently suffering severely from third party neglect, and while I see a lot of people blaming Nintendo, could it simply be that third parties just don't want to support a Nintendo system? Has Nintendo's pre-Iwata history developed so much animosity from third parties that it's unlikely the situation can truly be remedied any time soon?
Wii U is one of the worst performing systems of all time right now. And since devs are once again going for a multiplatform future - PS4/XBO/PC - simply by virtue of that being smarter from a business perspective financially, it's obviously going to make putting in the effort to make even limited cross gen ports difficult. And it's just going to get more difficult as people start dropping 360 and PS3 off their cross gen dev list, meaning Wii U games are going to have been developed with drastically altered engines and the like, once again raising the cost of porting and multiplatform support astronomically, and making it tougher as a business to support Wii U in any significant fashion.
With Wii U, the fault is all Nintendo's.