I personally think that fan demand
wasn't as big of a factor the time. It was far more of an important factor in Brawl and Melee, and Sakurai knowingly decided to go for characters who he could innovate with in moveset capabilities.
That probably explains why we got two shock characters (
and
, though the latter was a brilliant stroke).
and
were obviously chose because of what they could offer for their moveset.
was chosen because Namco. The most Pac-hate was largely because of his horrible Ghostly Adventures design, and subsided immediately once people saw how faithful he was to classic Pac-Man.
were chosen because they are practically everywhere. I don't think Sakurai had an option to
not include them.
This is my take on his choices.
I can agree with that.
Melee and Brawl had loads of hugely popular characters as newcomers: Bowser, Peach, Zelda, Ganondorf, Wario, Diddy Kong, Meta Knight, King Dedede, Olimar, Sonic and several others. Mewtwo as in Melee but cut in Brawl (though he was planned for the latter).
This game, outside of Dixie Kong, K. Rool, Ridley and Mewtwo, had no very iconic characters to add, as Melee and Brawl covered those voids already as mentioned above, so Sakurai went more on his own choices and what he thought he could work well in Smash. He's a game designer, after all.
Plus, I agree with @
pupNapoleon
in the post above. Sakurai knows about DLC, data dumps show both versions the game are designed to handle DLC and he chose Mewtwo, of all characters, to kick start DLC distribution. This might seem shady, but a very wise business decision. He knows people get excited for new characters in Smash and that can be exploited.
A game like Smash can always get more and more DLC, because you can always add something to it.
Let's be honest: characters like the Wii Fit Trainer or the clones wouldn't really sell well as separate content, so that's why they're included in the initial package. Speaking for myself and being a major Smash Bros. fan, I'll likely end up getting any and all character+stage DLC, but that isn't necessarily the case for everyone.
What is more attractive to a large public as DLC? Wii Fit Trainer or Mewtwo?
Sakurai did address popularity, but he didn't address all of the popular choices.
Finally, it is wise that Sakurai hasn't planned any DLC beyond Mewtwo (at least, at the time of the interview where he stated that). This ensures that additional content is made and released after the game has gained a large userbase and add legitimacy to it, as in, you're paying for extra content that was made beyond the initial full package.