VB Mario Kart is a video game planned for the virtual boy. the reason for its cancellation are unknown. but maybe it was due the low sales of the virtual boy.
There is not much information about the game. the only mencion we have of this existence of this are german nintendo magazine that ready VB Mario Kart as one of the game that were going to released for the virtual boy. no image or video of the game has been released. but there is a fangame called Mario Kart Virtual Cup that gives us an idea of what the game would look like