What's more likely? That third parties neglect the market leading console because the hardware is vastly underpowered compared to all the other formats or that every single third party has some grudge against Nintendo and intentionally avoids the market leading console entirely to stick it to them. I know that sometimes it feels that way but it's far too fantastic. EVERY third party feels this way? Every single one is willing to allow personal feelings to interfere with business? Bull****.
I think its because they're trapped in a vicious cycle where they believe only Nintendo games can sell on Nintendo platforms, so they either don't make any games or they just make crap, and then it ends up becoming a self-fulfilling prophecy, because if they don't make games or only make crap then no one is buying it and therefore people are only buying Nintendo games on that platform.
Some 3rd parties have seen success on Nintendo consoles, but the only way it happens is if they treat the console with respect and give a serious effort and then actually advertise the games once they're made. Very few 3rd parties actually do these things, so their **** ends up being a huge failure, but they blame Nintendo for that and don't acknowledge the real reason for why that is.
Ian, you say games can't be ported to the Wii because of the inferior hardware, but I think that's bullcrap. What do you have to say about games like Street Fighter IV making its way to the Iphone? The Iphone does not have cell-processors and gigs of RAM. If that game could be done on that device then I'm sure it could be done on the Wii as well. It just makes no sense at all. Many 3rd parties do have an irrational grudge against Nintendo hardware. Its either for the reason I pointed out, or because they still hold a grudge from the days when Yamauchi ran the company and bullied them around, or it may be because they see the consoles as "kiddie", or because they are jealous of the popularity and success of Nintendo first party games and would rather not compete with them directly because it makes their own efforts look that much worse.
These are the real reasons 3rd parties hate and avoid the Wii. If it isn't all of those reasons then its some of them, or some combination of them.