I mean why would an Italian plumber be the mascot for a Japanese company?
Because Mario Bro's is the first game for Nintendo and was the game that came with the console when you bought. Every time Nintendo upgrades their console they come out with a new Mario Bro's. Its like Sonic and Sega.
He's probably the most recognizable face of Nintendo, not to mention one of the most famous game characters of all time. I'm assuming that has something to do with it.
because Mario is the most successful game.