In all honesty, you could write a book on all the stupid reasons third parties didn't focus on Wii. From their completely ignoring it for the first two years of its life to the majority of them only making the cheapest and most 'casual' games ever in a sad attempt to steal 1% of Nintendo's market.
But all that aside, the main reason third parties are ignoring Wii now is three fold.
1) Piracy: Piracy is off the charts this gen. And Wii is the most pirated system. And just like the DS, most people (even pirates) buy the first party Nintendo games, but only 'hack' the third party games like Muramasa, Little Kings Story, No More Heroes or Madworld.
2) Handhelds: It costs 10x less to develop a DS or PSP game, with a much larger install base and less profit loss from pirates. The DS and PSP may be the most hacked systems, but they also have a larger profit margin for initial investment.
3) HD Consoles: While third parties probably could have made a huge splash on the Wii in its first 2-3 years, now the market is lost. Even Wii fans are hesitant to buy third party games and are switching to the much more reliable third party titles on the PS3/360. Heck, even I got tired of it and bought a PS3 recently. And I'm glad I did.
Nintendo is stuck in a hole. They can continue to dominate the market with just the profits from their first party line up. But they'll never have the full market penetration of a PS2 or SNES if they don't appeal to third parties. This is why I think they'll be releasing a more graphics focused console (like the GameCube) in 2012.
Six upcoming games you should look into:
