We could mention a few reasons why the Wii has done much better in sales than its comparatively more powerful rivals, starting with its unique (at the time) motion-sensor input system and the universal appeal of its games. Pricing also played an important role. Although the gap compared to the Xbox 360 and PlayStation 3 has decreased over time, at launch you could get a Nintendo Wii for up to $150 cheaper than the competition.
But just because the Wii is selling faster than the PlayStation 2 doesn't mean that it will also become the best-selling console of all time. The company has moved some 74 million units worldwide but is seeing a decline in sales as of late. Meanwhile, Sony has shipped more than 140 million PS2 consoles so far, and though monthly sales are not particularly strong compared to current-gen consoles, the fact that it's still on the market is quite remarkable.
Furthermore, Sony and Microsoft plan to level the playing field later this year with their own motion-sensing add-ons for the PlayStation 3 and Xbox 360, respectively. It remains to be seen if the Wii can continue selling at the pace it once did.