Because they are lazy developers: On PC, PS3 and 360, you have the power right there, easiliy available for your use. Even when NOT used in a optimal way, your game will look kinda nice.
The Wii is not like this: You have to struggle and get to use even the last drop of processing power in a perfect way to make your game look good compared to the other three.
...
Or maybe they didn't had money ;>