Dunno. 

Lack of effort in 06-08/09 meant that Wii owners honed in on Nintendo games and the occasional 3rd party game

Last year and this, 3rd parties have really tried, especially exclusivity (MHTri, Red Steel 2, Epic Mickey, Xenoblade, Little King's Story, Conduit, Goldeneye etc.), and this year looks good as well! (TLS, Conduit 2, de Blob 2, The Grinder, Pandora's tower, UDraw etc.)

So it isn't a lack of effort now, just a select few companies (EA!!!) don't put in the effort, and you don't get something for nothing...so what do they do? Make better games? No. They release poor mulitplat games (NFS, heck, FIFA doesn't even have Manager mode in!), make a good game in a poor genre (Dead Space Extraction), so instead of putting in the effort, they continue to put out poor games and blame the audience.

The Wii audience isn't stupid. They've learnt now. Only good games sell, and good games, well-advertised, do sell...

When Nintendo supports third parties, then maybe third parties will consider supporting Nintendo consoles. If Nintendo aren't willing to make the effort, then I don't see why third parties should try when they already have two or three viable platforms to put their games on, especially when they all have have higher attacth rates than the Wii. When someone does take the risk, then they don't meet their expectations and pull out. It's so easy to see and it makes more sense than making up weird fanboy conspiracy theories.

Not sure I agree with you there.  When you say "taking the risk", do you mean pulling out all the stops to produce a costly AAA title?  Because if so, I can only think of two third party examples on the Wii (Monster Hunter 3 and Epic Mickey), and they both sold very well.

Because they underestimate Wii's audience in every possible way.

That's it.

I think because graphics capability is the main concern of western developers, in the N64 era, it had a lot of support from western 3rd parties because it was the most powerful console, in the GC era, Xbox was the most supported console from western 3rd parties (a lot of those games were ported to Ps2 due to it's HUGE success), at the moment X360 and Ps3 are the most supported since those are the most powerful consoles, it's not about bias against Nintendo (maybe there is some but it's not the main reason for sure), to get another example, the PSP is a successful console and it's from Sony and I don't see it gets really that great support from western 3rd parties, some ports if ever.

Call of Duty: Black Ops, Prince of Persia: The Forgotten Sands, Red Steel 2, de Blob, A boy and his blob, Madworld, Dawn of Discovery, Little King's Story, NBA Jam, Silent Hill: Shattered Memories and many more are just a few examples of Third Party effort on the Wii. They are all good games, none of them sold relatively well and judging from the sales of these games and the ones from a few recent JRPG's (The Last Story/Xenoblade), I can definently see how third parties got the impression that most third pary efforts don't sell on the Wii. 

Epic Mickey takes the brand name of one of the most well known characters in the world and puts him into a game, while Monster Hunter is the third in a pretty popular franchise and they both did sell pretty well (as they should), but despite being serious efforts, neither of them set the charts on fire. Goldeneye is based on one of the greatest and most well known shooters in history and it only sold a million units, despite being given a significant amount of effort and advertising. 

The Wii audience is still a mystery, while the HD audience isn't and that's why third parties are willing to go all out on three platforms (PC, 360 and PS3) and that's because they've already proven themselves when it comes to making sure that great games sell well. 

Madworld - too much gore, black and white and WAY TO SHORT. I bought it anyway.

NBA Jam - average game. I can't even FIND A COPY. NOT EVEN AT LAUNCH HERE.

Silent Hill: Shattered Memories  - NO idea if a great game. BUT NOT RELEASED HERE EITHER. 

Dawn of Discovery - not out here never even herd of it.

hard to sell stuff that is crap or not distirbuted enough.

Playstation reaped alot of credibilty in a relatively short time, Nintendo needs time, and money, to win them back.

I could make excuses like that for every good game that bombed though. Also, I've seen all those games at my local stores and maybe if the Wii audience was a bit more aware of their upcoming releases, then it wouldn't take a game three years to sell 600,000 units. Also, Madworld is just your opinion, like my opinion that Red Dead Redemption is one of the worst games ever made, yet it received critical acclaim, it was just a matter of my taste.

They don't underestimate the Wii audience, they just cater to what the majority of the Wii audience buys.