To create demand obviously.
http://www.1up.com/do/newsStory?cId=3155501
Looks like only 600k units were sold in a week in NA, 400k short of the 'promised' 1 million that was announced in October, and a far cry from the 1.5 million number that was bandied about and I believe even noted by Nintendo execs in September.
I guess Nintendo got wise and realized pumping retailers with units wasn't smart after all (overstock = bad idea). Also, people have been throwing out excuses that it's a shortage... but really, if such a thing were real - Nintendo would not be giving up Wii units to those with defective consoles so easily and quick.
BTW, 600k is also the same amount Nintendo promised JP - if in fact they do get 600k - Reggie's claims of "The US will get most of the launch units!" are bullshit.