US region coding of Wild World is to blame.
Recently many Australian Wii owners have complained over an apparent glitch that prevents them from importing their Animal Crossing: Wild World character into the newly released, Animal Crossing: Let's Go to the City.
The cause of the issue was discovered to be a region mismatch. Australian copies of Wild World were in fact identical to the American release, including region coding. As a result, Let's Go to the City does not recognize the game on import.
Nintendo has yet to comment on the issue.