According to Fudzilla, the reason why Nintendo isn't offering GameCube backwards compatibility with the Wii U is because a few "key" GameCube titles will be offered via their Virtual Console store for their upcoming console.
In addition to that, the site's "sources" also lets out that we should expect Nintendo to announce "at least some popular GameCube titles as Virtual Console titles for purchase before the end of the year." Although they currently don't know how much Nintendo will charge for the GameCube games, other than "we do expect the cost to be more than that we can purchase a used copy of the title, in most cases."
Keep in mind that while this is a rumor that seems to have merit, it's still a rumor and nothing more until confirmed by Nintendo.
Here's to hoping that if this is true, we won't be charged an exorbitant fee just to play some of our old GameCube games. I'm guessing we'll know more about Nintendo's online plans and whether this is real or not as we draw near to the Wii U's November 18 launch.
Which GameCube games would you want to appear on the Virtual Console first?