They could make so much money off of it..
Example:
New! for 5 bucks (or however much that is in wii points) Mario Kart DS Leaf Cup (or something along those lines)
release a new set of tracks every few months.. it's easy money for nintendo.
I feel their selection of old tracks was hit and miss. Ghost Valley 2 I like because it's quick. I barely let go of the accelerator, just like when I played it on SNES. But there are so many other better tracks they could've put on.
I heard there's a new system coming out next year. Hopefully this new system will have a new Mario Kart that will include track downloads. When I would play Time Splitters 2 and Frequency online on PS2 you could upload your levels to your opponents. ...Wish they would implement something like that on this one where they would use the store to download free tracks from old games, just so you don't overload your storage.