Look at Mario Kart tracks themselves. The tracks actually based on in-game locations are a vast minority compared to the ones that only exist to be Mario Kart tracks. Also, look at the character line up, that refuses to acknowledge Mario's spin offs or even Wario's games. I don't think Mario Kart's developers are really interested in some "all star" line up of racers and tracks.
I'm not one of those wackos that wants Captain Syrup playable in smash, but she would fit really well in MK now that I think about it.