Because Pyrosphere would prove Ridley WASN'T a stage boss (or at least that Ridley wasn't THE stage boss). Most everyone thought Ridley wasn't playable already anyway. If Ridley was a stage boss on Pyrosphere, they'd have no reason to conceal it. However, if they have plans to show off Ridley later, of course they don't want people playing Pyrosphere, because he's not going to appear.
Literally the only reason not to show Pyrosphere is because they want to keep Ridley hidden a while longer. Honestly I think this is only more solid proof that Ridley IS playable.
I can think of a bunch more, but they all are tied to sakurai taking bad decisions (regarding at least my criteria) and while i recognize this guy have talent and probably his good decision ratio is way higher than his bad decision ratio. it doesn't mean that he is perfect
for example, I'm not sure if this game is going to get patched and balanced outside of its release date unless it have a game breaking bug, but if its not, i feel that is a little arrogant for him to think that he can achieve a good balance in the game withouth the game being out. the most balanced fighting games have had lots of years of fire testing. If i knew that he, or a team is going to tweak the game post release i would rest easy, but right now...
On the other hand, im happy the 3ds release get pushed back... less chances of having cuts.
oh, and people, don't expect a Ridley reveal this e3, the Sal leak was true. it doesn't mean that Ridley chances are over, the leak was regarding characters that were planned to be shown on e3, but if anything he will be a secret character a la wolf.
Actually, right now i'm most worried about Robin : /. although there is still a little chnce that the second part of teh elak fas fake, but chrome seems to be with a shoe in...