Ridley is basically the only character with a large fanbase outside of Smash Bros. for a veteran series that is not yet playable. Metroid is also quite popular and well-known with gamers.
Face it Ridley is the last all-star not yet playable. I can't think of a single character that is as well-known and important from within their own series that has not yet been made playable like Ridley. K. Rool, while important within the DKC series, was not with the Donkey Kong series from the beginning (like Ridley) was and may be fading out of importance. Bowser Jr. only appeared in the last decade for the Mario series, and Toad while he started important, basically faded into obscurity after the SNES (and only got a resurgence in importance after NSMBWii).
This is false, but whatever.
If you think Metroid is bigger than the likes of DK, or even Wario or Kirby, well that's just funny because it's factually wrong.
If you don't think K. Rool, Dixie, Little Mac, Mewtwo, Roy, Mona, or even Samurai Goroh on a good day compare to Ridley (I'm assuming we're not including 3rd parties), then I can only assume you only visit/listen to places that generally emphasize the same few series (Mario, Zelda, Metroid, Smash) when it comes to Nintendo. That'd explain the myopia.
The "hasn't done much lately (how lately is lately enough?)" and "hasn't been around as long as Character X (how long is long enough?)" justifications fit pretty well under the whole "vague as hell" bit I mentioned before. Guess as long as it fits your own wacky definition, it's okay, right?
@Star: I thought it was gonna be some YouTube link or a PM with pictures and characters and whatnot like some big presentation, don't know why. Guess this is pretty solid too though, but maybe I should just wait until someone else who knows as much as you do shows up and I can just follow whatever you two would discuss.
edit: oh wait, I tried Googling some of the characters you mentioned and got this:
http://www.hardcoregaming101.net/dragonquest/dragonquest.htm
WHY CAN'T YOU KIDS USE GOOGLE
edit2: CB, I know what you think, I also know you're completely unaware of all but the most vocal opinions in regards to, well, everything. To be fair, that seems to be par for the course for most people who obsess over this stuff, so consider yourself enlightened compared to the random fan who asks for Kool & the Gang