So why do we get Wii Fit plus / Mario Galaxy but no new Mario Kart?

[SmokedHostage]

EssosDeeos said:
Maybe Wii Fit sales are more about the profit the price with the balance board gives hence no problem with a sequel?

Wii Fit Plus is expanding on what the first one did.  Pretty much, it's adding a routine option and Motion Plus.

Would a new Mario Kart benefit from Motion Plus, and could it be expanded upon that it's pretty much a game in its own right?