We have done this Iwata Q&A article to death...it's been gutted a thousand times over by now. While we've brushed on this topic a couple of times, we haven't gone and posted the direct bits concerning this topic. Long story short, Iwata was asked why game sales are so much bigger in December when it comes to North America.

The largest reason is Christmas. Americans buy a lot of presents during the Christmas holidays. Christmas gift guides, which I prefer not to call flyers but rather catalogs, usually circulate at the end of the year, and products advertised favorably there usually sell extremely well.

Check out the full answer here