Third parties fucked up software-wise, but Nintendo massively fucked up here, too (and not just in failing to push and prod and pay out for more strong third party support out of the gate). I'll be frank, all the problems the Wii is seeing right now are Nintendo's fault.
The company essentially ceased GameCube development in late 2004 to mid 2005, and from the moment the Wii was unveiled Iwata and his cronies were hyping up its potential for easier, quicker development due to the similarity (lol) architecturally to the GameCube. They had well over a year with which to devote their vast first and second party dev resources (not to mention using some of the cha-ching to fund some outside stuff) in order to prepare for the Wii's launch... but in the end, they had practically zilch to show for that.
Now another year has passed (meaning it's now been three years since the GameCube was killed off) and... what? We've gotten Mario Party, Mario Strikers, and the just-announced Mario Baseball, and precious little else. They could have, and should have had a pretty damned impressive lineup of games over the past year and a fairly hefty lineup ready to go for 2008, but it hasn't been the case at all. It's been a pretty pathetic performance all around.