For how popular the DS (systems) are, the games for the most part have a very short shelf life. This is why you have seen re releases of certain games. It is almost like they think all of the DS systems that are ever going to be sold have been sold, so there is no need to keep a game in circulation.
It also depends on where you live as to what type of stock level for a certain game is. But there are tons of games that if you want, you will have to really look around to find it. I am always surprised that for how big EB/GameStop are, they don't really fill this void in keeping the harder to find games in circulation via the used market.
On past Nintendo sytems, their games remain around through the duration of the system. Even taking a price drop at some point. However, look at Kirby CC, Yoshi Touch and Go, Advance Wars ect., you actually have to really look to find these. (Unless you live by a store that never sold through)
I think with a system growing the way the DS is, it is a huge mistake to take games out of productin so soon. This is also a reason that I tend to buy most decent DS releases when they first come out. I really don't want to go looking for them later.
