NATT44
Turtle King of The Internet
Towns Folk
I believe that Mario Kart's popularity is due mostly to its accessibility. The game is exciting to play and easy to learn, especially compared to more complex games such as racing sims. The low entry-level difficulty means anyone can play, and there's enough higher-level content to hang on to a more skilled fanbase as well.
It helps that Mario Kart is a relatively unique formula - there aren't a lot of other party racers that can compete at Mario Kart's level at this point.
It helps that Mario Kart is a relatively unique formula - there aren't a lot of other party racers that can compete at Mario Kart's level at this point.