Two reasons.
1. Its bundled with a lot of systems in North America.
2. It effectively has no competition. While there are plenty of other platformers out there, Super Mario still has the "genre king" trump card. Since it was the series that established platformers popularly (and Nintendo have protected its brand with very few, high quality entries), it basically remains the #1 choice for anyone looking for a platformer, giving it huge legs.
Mainly its the bundle, as NSMB sold 422K of that 537K in NA. But then again, Nintendo basically doesn't bundle anything that isn't a "genre king," because they have the highest potential to push hardware sales.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.