TheThoneStar has a point. A lot of people buy DS solely for Pokemon. They also buy other games, but that is the reason they get it above PsP.
Nintendo can't afford people to get tired of Pokemon, and thus only 2 games over 5 years works fine.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS