Well, I bet Nintendo makes more money per DS sold, in comparison to the money they get everytime they sell a Wii. Plus, there are far more AAA games on the DS assuring Nintendo's pocket becomes even fatter. Basically.....they don't have to put much effort into making money at all.
However, ignoring the Wii would be a disaster, but then again....they wanted to have 4 million out by the year's end, and they have sold 3.17 mil. I think by years end they'll have 3.5 million around the globe, which puts them ahead of schedule to sell 6 mil for the fiscal year.