| The reason Nintendo did this was because the Japanese Super Mario Bros. 2 was decided to be much too difficult for us poor, dumb Americans (admittedly, it was quite difficult - its 1-1 was analogous to a theoretical 9-1 in SMB, for example), and the fact that it was using the same engine as the original (with a very few additions - such as the slightly-differently-colored poison mushrooms which certainly wouldn't fly with Americans, given that so many are color-blind and, until recently, color TVs were still a bit of a luxury in the always-technologically-delinquent US) made it seem more like an expansion than a new game.
That said, it made for a rather nice Mario game anyway, and it's still the only one like it (face it, the rest of them are basically just variations on the first one when you come right down to it).
Oh, and having a gender-ambiguous dinosaur in it doesn't hurt it any. :) (Although Nintendo currently claims that Birdo is a she, as did they for the Super Mario Bros. Super Show, the SMB2 manual stated something to the effect that "Birdo is a he, but he thinks he's a she, and so he lays eggs." I always found that admirable for some reason...) | 
