Why Are Mario and Mr.L Plumbers?
First of all Mr.L was an alter ego for Luigi Mr.L himself isnt a plumber so much as an evil robotics expert who works under Count Bleck. However Mario and Luigi dont have much of a reason to be plumbers. The most accurate reason was that Nintendo wanted to make a story for the mario brothers to fit their first game. In the Game Mario Brothers it was stated that two Brooklyn orn plumbers with an Italian origin were inspecing the local sewers when crabs and shellcreepers took over. Thus the easiest way to explain this was to say that this was their job, since then Mario and Luigi have been plumbers. This also lead to their discovery of the Mushroom world, as another game states they were sucked down there by a secret warp zone while trying to unclog a drain. This statement makes it easy to use the pipe levels and themes in Mario games.
