http://www.sourcegaming.info/2016/02/21/nintendodream3/
—What led you to decide to make a Geno costume after all this time?
Sakurai: To tell the truth, it’s because I wanted Geno to be a playable character. He has a gun for a hand, and I think he fits in really well with
Smash. During
Brawl, I thought it would be great if I could add him, but in the end it didn’t become a reality.
—Because of that, you added him as a Mii Costume this time?
Sakurai: That’s right. He’s very popular. When talking about older characters, Geno always gets a lot of requests. He didn’t appear as a fighter, but I was able to achieve something kind of close to that as a Mii Costume.
He says right here that he wanted him to be a playable character. Plus, that's not the only reason he tried to make him playable. It's for his fans as well.
It means that there aren't any "rules" to how a character is chosen for Smash bros