Because about 80% of the users on this site have decided the Sal leak is canon. I want to be able to predict newcomers again without being told I'm wrong because Sal didn't say so.
The Sal leak IS legit. There's no doubting it at this point. Look, I refused to believe the Sal leak, and rightfully so. There were too many inconsistencies for me to fully believe it. But the latest leak revealed the names of Mii Fighter and Goddess Palutena completely accurately. And now we know Rhythm Heaven is in Smash Run, as well. We can doubt the possibility of who the Rhythm Heaven rep is, but we've got a Rhythm Heaven rep, end of story.
A non-leak rep won't disprove the leak, it just proves that the leak isn't the entirety of the newcomer roster.
EDIT:
Doesn't Rosie herself say though that Sal's source doesn't have it all? The entire roster of newcomers? Considering Sakurai's statement about characters having been picked early in the game's life in production. It's logical that Sal's source had a document that had some newcomers on it, but not ALL of them.
The argument is that Rosalina was revealed "in between" leaks so it doesn't matter. A non-leak newcomer would prove that there are additional characters we don't know about either.
I'm expecting atleast one out of the hype 3. Mewtwo ridley and k rool. Mewtwo is one I'm sure is in. K rool and ridley are iffy on the other hand.
Mewtwo is the easiest, because he's a veteran. They just have to prioritize Mewtwo over Jigglypuff. Considering the character popularity, and the fact that according to the leak they don't know if they should cut Lucas or Ness, its not hard at all to suggest that they may very well consider cutting Jiggly this time in favor of bringing Mewtwo back.
I still think Ridley is in the game, but if he's hard to develop, they might be planning for him to be post-release. K. Rool doesn't have anything against him at this point, and the Kremlings heavily point towards his possible conclusion. One major factor, though, is that K. Rool has not appeared in a game since Brawl, and all other characters have appeared in a game since Brawl. That said, neither have the Kremlings, so it also helps K. Rool, too. K. Rool could also end up as a DLC character if they're having problems getting him finished.