Producing a product as complicated as the Wii isn't quite as easy as having truckloads of raw materials dropped into your factory and then having Wii systems drop out of a chute ... To produce the Wii dozens of companies produce components which are then shipped to Nintendo; these components are then combined to create either an intermediate or the final product. Nintendo generally chooses components that have multiple suppliers but they can still be put in a position where one or two components can limit their potential supply.
At the same time I'm not too sure that Nintendo anticipated success on the level of the Wii; consider that the Wii is selling at a much faster rate than any Nintendo console has ever sold at this point it its life. Nintendo (being a cautious company) is probably planning on increasing supply in several small steps rather than a couple of large stages in order to prevent over producing the system ...
