If sexism didn't exist, there would be more recognizable female figures in the land of games in general, making more characters eligible for Brawl. I wouldn't blame the smash bros. developers, I would blame gaming developers as a whole, since Smash Bros. is merely a collection of iconic characters from nintendo's universe.
There is unfortunately a low number of iconic female reps in the gaming world. I mean, if the roster were specifically half female with all characters from nintendo games, it would be filled with characters that were put in because they were female, not because they're iconic. I don't want that, personally. Affirmative action: NOT IN MY BRAWELZ!!!11
Hopefully in the future we'll see more iconic female representatives for the big N that can smash with the bros.