—Some guy we forgot
Because Ridley is too big. And besides Samus, Anthony is the only character to survive the events of Metroid: Other M, but since being dead doesn't seem to matter, he has competition in the form of Adam and the Hunters from Metroid Prime. But the problem is that Sakurai clearly doesn't care about the Metroid Prime series, or else Samus wouldn't have gotten a suit upgrade, the Pyrosphere wouldn't be playable, Ridley wouldn't use his Roidley design, and Dark Samus would be playable. Nevermind the fact that the Prime series is one of the few spinoffs Sakurai acknowledges at all, he should be begging to add in more characters from it. But I digress. Anthony's only competition now is Adam, and no one wants to see "Adam Authorizes the Battle." So therefore, Anthony is the only viable Metroid rep.
Also, he'd be the first playable black character in this game! Don't say Donkey Kong is black, you racist fuck.
On the Other Hand...EditIf Smash Bros. truly is a competition to see who's strongest, as seen with the death of Chrom, and to allow them in, then Anthony wouldn't stand a chance, since he lost to Ridley. His only redeeming factor against the Riddler is that, once more, he's not too big.
Also, Sakurai is biased against Metroid and Donkey Kong, so they probably won't get newcomers. Maybe next time, Anthony.
Add yourself to the list if you support Anthony Higgs as a playable character in Super Smash Bros. for 3DS/Wii U.