I think that Nintendo made a smart move. Simply taking the original game and upping the difficulty wouldn't have satiated the gaming populace's need for a new Mario game. The die-hards would have embraced it, certainly, but the entity that is 'casual gamer' was rearing it's head at the time, and the change to something fresh kept that segment of the game playing public happy.

Besides, I think SMB 2 (US) was a fantastic title. I like it much more than I liked the original, but then again, I didn't get the original with my NES. I got the DELUXE set. :P