Heavy lobster anyone?
Also, I don't understand why people want Ridley so much.
"We want Ridley!"
He's too big.
"Put Ridley in Smash!"
He's a boss character who fights on your side. However, you can vote for other characters on the Smash Ballot.
"LETS VOTE FOR RIDLEY!" -.-
TL;DR: Metroid is a series that barely squeaks out into the top 10 of Nintendo's best selling. The series itself has a very vast and open feeling to it, being a Metroidvania. Due to this, there is very little that has been done for character outside of the canon Manga and Other M which tried to cram a metric ton of the manga into a very short game and handled it very poorly.
So thus, when it comes to characters that exist for more than a game or two, you really are limited to three characters.
/
: The Iron Woman from a tragic past that only gets darker who works now as an intergalactic mercenary who has a terrible record of concluding a game without blowing up the planet she's on.
Mother Brain: The primary antagonist to the series by merit of being the lore organizer of the Space Pirates as a leader to them and being the reason why the Chozo race have gone extinct. Because her appearances are limited to: A Brain in a jar or that brain put atop a massive dinosaur-esque body, she clearly does not fit that well into the Smash formula by design alone.
And finally...
Ridley: The foil to Samus as a character by bein the person who traumatized her by feasting on a buffet of humans, including her parents, infront of her very eyes. Often taking place as the Penultimate Boss, Ridley often symbolizes what you strive to prove you can beat to be the umatched powerhouse ready to end off the game proper. His design is one that has actually been used repeatedly in Smash for various roles and is the best hope for Metroid ever getting a character who isn't a version of Samus.
Because the next best character in terms of chances and popularity is
Dark Samus, the sentient murdertron 5000 whose actions don't differ too much from Samus in that regard but is made out of the remnants of Samus's phazon suit from Prime 1 AND THUS IS EVIL AND MUST BE STOPPED. Effectively ending up being like Shadow to Sonic by being a mirror of your abilities through an Edgy filter while being a more bland character.
Edit: In summary, Ridley is really the only choice for a Metroid character that isn't a version of Samus in Smash who is a very iconic and intimidating villain but ironically it is those very qualifications by which Sakurai believes that he works best as a boss because unlike the likes of Bowser and to a lesser extent Ganondorf, Ridley's fear-mongering is due in part to the scale he is always depicted in comparison to Samus. This is made worse in his design outside of Metroid 1 and Other M where he is REALLY lanky so his arms and neck would be skinnier than Greninja.