Because purely eccentric and off-the-wall picks for an all-star Nintendo crossover fighting game really grabbed people out of their seats and threw them into the stores, right? I know Pichu, Game & Watch, Ice Climbers, Dr. Mario, Young Link, Falco, Toon Link, ROB, Wolf, Wii Fit Trainer, Villager and Rosalina really told me that the developers were aiming to earn my money. I mean, all those characters I asked for? Shoot, who WOULDN'T buy their respective Smash games, right?
All kidding aside, Sakurai knows what people want and, at the end of the day, people develop games to satisfy their fans and not their strange, off-the-wall character OCD, y'know? The stakes are way higher this time, as well. We can't really just handwave all the eccentric picks anymore. It's not just do or die at E3 for playable characters, it's do or die for Smash as we know it, and more importantly to Nintendo, the actual Wii U itself. Ridley is just one symptom of a greater problem.
Certainly true, a lot of people aren't going to care that Ridley's in the game or not, but they're not trying to cater to us hardcore Smash fans that're going to buy the games regardless. They're catering to the Metroid fans that would say 'Wow, the new Smash has Ridley? I should really play that game.', the Donkey Kong fans that would say 'Wow, the new Smash has King K. Rool? That's awesome!', the Golden Sun fans that would say 'Man, I remember Isaac! I gotta play this game!', the Kid Icarus fans that would say 'Hades confirmed to be in this game? Yes please!'.
Etcetera, etcetera, ad infinitum. Let's be honest, character picks like Wii Fit Trainer, Rosalina and (god forbid) Chorus Men have got more people cocking their heads in confusion than out of their seats in hype. We need more villains, more characters that people are looking for, asking for, and less off-the-wall character choices. There's a reason why they swept the Wii Fit Trainer reveal under the rug at the end of last year's E3, 'cause even Sakurai knew people were going to question his sanity at that point.
EDIT: Seriously.
Watch this. The applause and laughter don't even overcome the audio on the clip itself. People are laughing and clapping 'cause they're wondering if it's some kind of joke.