I think we've given more than enough reason why developers would choose not to develop for Wii.
Lets say you were a third party back in 2006, waiting for Nintendo's new system. You know what Nintendo did? They said: "FUCK YOU". Instead of giving third parties Wii development kits they told them to work with GameCubes instead. Nintendo said: "So you bought all those expensive engines and shit? Well that's not going to fly on OUR system."
We just sold a million of these things so here's your turbo boosted GameCube and some motion controllers developers, have phun! Capcom: "So yeah we're making a Resident Evil lightgun shooter game while we figure this shit out." Nintendo: "Oh yeah cool, show it at E3 or something." *E3 2007* Nintendo: "Lol wut, sorry Wii Zapper, oh I'm so sorry probably forgot to tell you right!" Namco Bandai: "So yeah our Wii Support is quite weak but..." Nintendo: "We want to buy you!" Scamco: "Wait wat?" Nintendo: "Buy buy buy, Japanese do you speak it motherfucker?" Scamco: "Here, take Monolith" Nintendo: "Cool, thanks!"
Factor 5: "So yeah we pitched Kid Icarus and Pilotwings..." Nintendo: "Not gonna happen dude!" Factor 5: "Remember... our time together, Rogue Squadron?" Nintendo: "Yeah, well you pissed the bed" Factor 5: "What about Kuju and N-Space and..." Nintendo: "Dropped like a rock!" Nintendo: "OH WOW, we broke every freaking record on the planet, lets hurry up and finish this Music tech demo or some shit and port Animal Crossing and go on a holiday!"
Nintendo: "Oh seems like our system in the shitter now, how about we get New Super Mario Bros. Wii out and break every record again?" Activision: "I'd like to see you try, we have Modern Warfare 2 coming up!" Nintendo: "Lol Modern Warfare call of duty some shit, we'll take that bet!" Nintendo: "OH WOW we sold more than every third party title ever did 'combined', we're so fucking awesome right now. Too bad all your shit bombed developers, we really tried and totally didn't ignore you at 3/5 E3 shows <3."