One person emails Sakurai requesting that instead of having unlockables, that everything should simply be open form the start. Sakurai says that having secret characters is one of the fun parts of the Smash Brothers series. He says that in this digital age (this is not a direct translation, just my rewording of it), information moves far too quickly to be able to keep all of the characters a secret before the launch of the game. With Melee, in the weeks leading up to the game’s release, a new character was revealed each week. Not every single character was revealed, but some were. He says he wants to keep some characters secret until the “very end,” just as he did with Melee.
Related to the above, he mentions that Solid Snake is more fitting as a unlockable character, seeing as how he is a guest character, rather than being immediately available for play, even though we know he’ll be in the game.