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 mention we have of this existence of this was a German Nintendo magazine showing VB Mario Kart as one of the game that were going to be released for the Virtual Boy. While no images or videos of the game has been shown, there is a fan game called Mario Kart Virtual Cup that gives us an idea of what the game would look like.