Barnyard, Chicken Little, Cars, Happy Feet, etc. are available for all of the consoles. They sell just because they're based on hot movie titles.
There will be tons of games for the Wii.
The equation is:
more consoles sold = more games sold
more games sold = game makers wanting to make games for that system
Game Cube was #3 in the console game at the time. If you were a game maker would you want to sell 200,000 of your game to Game Cube owners or 20,000,000 to PS2 owners?