Also, while not set in stone, there is the possibility that Brawl didn't have the luxury of: DLC.
Yes, we know the potential evils of such a concept (though Nintendo so far has done a good job of how they handle it compared to other companies) and that Sakurai wants the game to be 100% complete, but chances are, especially with Nintendo undoubtedly holding him to that 2014 launch window, he won't have enough time and/or resources to get absolutely everything he wants in the game, characters or otherwise.
Not to mention Sakurai has stated that patches will be provided should something pop up, so clearly there is updating systems on the mind. It'd also give a shot for the more unlikely characters, like Roy for example, to have a second chance should they not make it to the finished product.
But then there is the problem of complaints from people who don't have DLC characters or what have you fighting against those who do online, so it's a double-edged sword to be certain.