So today on Kotaku there is an article about how Sakurai chooses the roster and it's almost like he's purposely being a troll.
http://kotaku.com/smash-bros-creator-explains-how-characters-get-on-the-1644208199
If it's tl;dr I'll break it down for you.
1. "We conduct a character popularity poll both domestically and internationally." Okay so I'm pretty sure Ridley would of been there somewhere...
2. "Does the character have something inherently unique about them? Can that be used to make Smash Bros. interesting?" what the hell does Dark Pit or Lucina bring in this aspect? Eh?!
3. Characters from a series that has no future are rarely chosen. Also, fighters that switch models are a different matter." So... Mother 4, F-Zero WiiU, Metroid WiiU, Starfox WiiU confirmed?! This guy man I swear...
4. "Basically, any character is accepted if I believe it works [in the game]. On the other hand, even if a character is unique, if they overlap with other characters and aren't unique as a fighter, they are rejected." Cloooooooooones...