Developers can't do what they do on the PS3 and 360 and sell well on the Wii.
Why so many test games on the Wii? Its because Developers don't know what to do on the Wii, on the PS3 and 360, they find a formula, like Zombies in shooters and run it ad nauseum, Zombies, Zombies in Space, Zombie Nazis etc.
But on Wii their attempts don't really work, they tried it with On Rail shooters for example, but most on Rail shooters don't sell well on Wii. They tried Minigames, but most non Nintendo minigames flopped, the developers find that the system on the PS3 and 360 doesn't work on the Wii.
In addition Developers don't like Nintendo, they don't like to compete their games against Nintendo, Sony and MS consoles don't have that problem, its safer for the third parties
Predictions:Sales of Wii Fit will surpass the combined sales of the Grand Theft Auto franchiseLifetime sales of Wii will surpass the combined sales of the entire Playstation family of consoles by 12/31/2015 Wii hardware sales will surpass the total hardware sales of the PS2 by 12/31/2010 Wii will have 50% marketshare or more by the end of 2008 (I was wrong!! It was a little over 48% only)Wii will surpass 45 Million in lifetime sales by the end of 2008 (I was wrong!! Nintendo Financials showed it fell slightly short of 45 million shipped by end of 2008)Wii will surpass 80 Million in lifetime sales by the end of 2009 (I was wrong!! Wii didn't even get to 70 Million)