Because the Wii U hasn't been out for 2 years?
It launched November 2012. Which makes it about 17/18 months old.
6 million/ 18 months = 4 million a year
4 million x 5 years (usual length of a Nintendo gen) = 20 million
(assuming we're seeing a steady rate of sales with no change, which is doubtable. Mario Kart etc will either raise the baseline, or it sales will slowly fade away to nothing)







