For the same reason you don't see link, nor star fox, nor kirby, nor pikachu or anyother else: becuse is named MARIO Kart. Not NINTENDO kart.
Why she did appeared in SMRPG? That game was made by Squaresoft and they had "that" idea. MK was made by Nintendo. Besides that game and the smah bros franchise, what other games you saw that to repeat?
Diddy Kong I agree shouldn't be there because he belongs more to the "Donkey Kong country" games but I guess that they included him because HE'S related to Donkey Kong which belongs to the mario universe (DK was the first enemy of mario before bowser). So he's sort of a "missing link" to those games. Or the DKC games are spinoffs of the mario games, could be.
And R.O.B? meh that was just fan service crap.
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