Am I the only one here who thinks Shadow is overrated? I mean he was cool when I was 7, but now he's all "argh, i don't say anything, i only talk in this low raspy voice, i'm so cool because i have red highlights and i'm like sonic but i'm black."
Also keep in mind that Shadow is a minor character in the Sonic series. The last time he had a major role was Dark Brotherhood.* That game came out 6 years ago. He's only appeared in spin-offs because he's a staple character, like Wario** or Daisy. Shadow would've been alright for an Assist Trophy, but not as a full-fledged character.
Just because Shadow has a viable moveset does not mean he should be a character. You know who else could have a good moveset? Tingle, that short elf-manchild from Zelda. His B could be that he throws a rubee, his Side-B would have him charge into the enemy. His Up-B has him create balloons for him to float, and his Down-B would be the Tingle Bomb. His Final Smash? Koolimpa Dance. But you don't want Tingle in Smash Bros, do you?
So no, Shadow should not be a fighter. He's in Super Smash Flash though if anyone wants to play as him.
* - I didn't forget about Generations, but he's just a boss fight. He doesn't really do much for story.
** - Wario has his own series of Wario Land and Wario Ware, but he hasn't been in a non spin-off Mario game since Super Mario 64 DS.
I'm not really sure where you're getting at, they have tons of games, it's just that some companies don't want to develop for Nintendo consoles. Nintendo has a family-friendly target audience, and unless you're developing for that kind of biz, then it'd be a horrible idea to make games for them at a marketing standpoint. Besides, I honestly don't think Nintendo cares what people make as long as it follows their guidelines. (no nudity, no religion, some other crap.)