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Even Nintendo had trouble selling more hardcore oriented games like Metroid Prime and Sin & Punishment and Xenoblade on the Wii, even with a 100 million userbase. Like Metroid for example saw basically no bump from the GameCube days. NOA just bailed on Last Story and Xenoblade, but thankfully were pressured into at least giving Xenoblade a limited release by angry fans.
Third parties did support the Wii, just with the types of games that were actually selling on the console though -- things like fitness games, party games, dance games, etc.
If Nintendo had an IP like Halo or Uncharted that they made themselves and sold millions and millions of copies, then I think third parties would be more inclined to support Nintendo platforms. It's become a demographic problem as much as a hardware issue.
Sony/MS are simply are much more attractive option for third parties. They know hardcore gamers will be there, because Sony/MS spend virtually all their marketing dollars on that audience and tailor their console designs for that audience first and foremost. Sony or MS would never even consider making a purple console for example. Nintendo makes their consoles for the family audience first and foremost and most of their big IP are family-oriented titles ... that just doesn't speak to the developer of say, Metal Gear Solid or BioShock or Grand Theft Auto.
Mario Kart selling 20 million copies doesn't mean much to them. If it was Metroid or Xenoblade or Bayonetta 2 selling 10 million copies on a Nintendo platform, I tend to think that would draw a lot of third parties though because they would be confident there must be a hardcore audience there.
But if a developer is looking at the Wii/Wii U and seeing that even Nintendo has a hard time selling non-family-oriented brands on the system, what message does that send to say the GTAV designer?
