As other in the topic have already said, Nintendo wants most Wii's to be gone from stores when the Wii U comes out anyway. This way, they'll be no confusion with the name and with people buying the cheaper Wii instead. If the Wii U is the only hardware in stores and people want to buy Wii or Wii U games, that will be the only Wii system they're able to get.
Even last year with the terrible lineup the Wii had, it still managed to sell over 2 million system in North America during the holidays. Even with Wii sales even lower this year, the system could still do over a million during the holidays as well. Good reason for Nintendo to try and kill the Wii hardware off now so there will be no cheaper alternative this Fall so that extra million that would have bought a Wii will have to buy a Wii U instead.