That's the thing. Sakurai just outright kept Ridley from being playable just because of moveset.
Swordsman? Eh. Telekinetic, alchemic... Whoop-dee-Tingle-doo. Nobody cares what he's powered by. Explosions? Link, TL, Jr, Samus, Megaman, Zelda, Duck Hunt and a ton of others have explosives. Isn't presented in a wholly unique way either. Earthquakes? DK does that already, and I somehow doubt we'd get the ability to cause POW block level Earthquakes without a massive charge time. At that point it's a minor functional difference from Robin's Thoron. Healing is covered by the WFT. He brings nothing unique or significantly interesting to the table.
Shulk is the only character, besides Palutena's limited Lightweight custom, who can actively change the whole matchup with a press of the button. He's everything all in one, but only one thing at a time. Now please, I don't think you could successfully argue that Shulk isn't unique to anybody.