Well, hey, let me apply some Smash fanbase logic to Mario Kart. What's good for the goose is good for the gander, yes?
"You guys want Bowser Jr., Diddy, and Birdo in the game? You're so entitled! Stop whining! You already got popular characters with the Koopalings! Let the Mario Kart team exercise their creative vision; it's their game!"
"The Mario Kart team is loaded with geniuses. They added Baby Rosalina and Pink Gold Peach, characters who you didn't know you want, but were really iconic Mario universe characters just the same. Learn to love 'em."
"This game is soooooo beautiful and is literally flawless. How dare you want a better roster?!?!" *goes off to complain about the gameplay, items, modes, music...*