I think Porting to an external platform is not as easy as giving away few bucks.

Engine must be capable to be ran at the platform
Engine must be tested on the platform
User must understand the platform
User must be capable to test the game on the platform

Too much for average user, so to still get the Wii porting;
Additional edition, "Professional Plus" with Wii Porting Capability.

(and then we notice that Microsoft products like DirectX aren't supported by Nintendo Wii.)