The Nintendo Wii, a hugely successful console, is experiencing a sales slowdown. Wii sales are down 21 percent — the biggest decline since the console launched in 2006. But why?

It is important to keep in mind that the Wii has been a juggernaut, running rings around the competition. So even when Wii sales drop by 21 percent, Nintendo is still able to have its third best year ever.

The decline in Wii sales, however, does beg the larger question of why.