Not to mention that Sheik and Ocarina of Time are much more prominent in gaming as a whole, and Ocarina of Time is arguably the greatest game ever made. There is reasoning behind Sheik's inclusion.
Geno does not have that going for him. As I said before, Geno is a character that i'm sure everyone finds to be awesome, I myself am included, and I think he would be cool. But should he get in over several other characters? Absolutely not.
I hardly consider him an all star, and neither does Nintendo apparently. Really, Smash Bros. is a paradox, because you have to appeal to fans with the popular characters, but you have to consider the players who know next to nothing about Nintendo. So you have to inform them through other means. This can be done through trophies, stickers, etc...
Geno honestly would be cool...but he isn't relevant, and Nintendo would not get as much out of him as say Megaman or even Pac-Man. It wouldn't even be worth the money to use him in the first place.