Mario Kart Wii also saw sellouts throughout 2008. Supply wasn't really plentiful until about a year after launch.
It's all about the plastics.
Wii Fit: Special plastic board.
Mario Kart Wii: Special plastic wheel.
NSMBW: Special plastic box.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.