Why the hell would you want ridley in the game?
*He could be designed to be basically the anti-little mac: great air game (including aerial grabs) that allow him to be a menace in the skies, but his land attacks are terribad, and he's slow on the ground. To combat the fact that being air-focused means he will most likely have an amazing recovery, I would also make him very easy to juggle by having a fast fall rate, and make himself focused more on combos than straight up kill moves. He would be extremely unique, and fill a niche no other character has.
*Metroid is an important Nintendo series that is likewise amazing, and it is a travesty that it only has one true character to come from it for over three Smash games (Having two different versions of Samus doesn't cut it for me, even if ZSS is nothing like Samus)
*His design is really cool. Purple space pirate dragon. Who /wouldn't/ want to play as that?
*Ridley is bar none the most important character in the entire Metroid series outside of Samus herself. While he is never the main antagonist himself, the fact he appears in almost every single game (he missed three games) makes him a much more recognizable villain than Mother Brain Dark Samus, or any other villain in the series. Samus and Ridley's personal vendetta against each other is always an important side arc in the games. If we're going to include Bowser and Ganondorf as Nintendo all-stars, then Ridley should get in for basically being Metroid's Bowser.
*Part of Melee's opening shows Samus fighting a Samus-sized Ridley, which 6 year old me wanted to be able to replicate. I never heard of Metroid in my life before Melee, but that one particular scene struck with me for years. It's now over a decade, and I want to be able to play that fight out like it has been teased for so long.
*Other minor reasons which are part of those listed above or aren't really important.