Why not? It's a pretty spectacular roster; I'm actually surprised we have so many characters. There are 49 in the leak, correct?
S64: 12
Melee: 26
Brawl: 35 (or 39 if you count Pokemon Trainer, Zelda/Shiek, Samus/ZSS as separate)
Smash4: 40 officially confirmed so far, 51 in the leak, counting the 3 types of Mii as separate. That's a lot of characters!
Sakurai has talked about the exponential difficulty with increasing the roster by 1 character -- all matchups need to be tested, all game-modes and items etc... Plus they're two different games (3ds and wiiU) that both need development and testing.
There's a whole lot more to the game than the character roster, and honestly at a certain point it doesn't add much value to the game to throw in another character.
The whole "all-star mode is unlocked" thing seems like an unlikely complaint to me. It looks like we've just seen screenshots/clips of the game that Nintendo sent to the ESRB. The shots might have nothing to do with each other. A screenshot of the menu with a "locked" icon is just an example of the menu, the roster screen was probably the full roster so that the ESRB can know there's no hidden character that might change the rating. Just my guess.