Because Bowser paid him to.
Why wasn't there a black or white Yoshi in Yoshi's Island?
"So, they''''re showing, on television, the detergents getting out bloodstains. Is this a violent image to anybody? Bloodstains? I mean, come on. You got a T-shirt with bloodstains all over it, maybe laundry isn''''t your biggest problem right now." ~~~~Jerry Seinfeld