It doesn't make any sense. I mean the character isn't even close to being considered a PlayStation All-Star. We all know Mega-Man went to trash when he entered the 3D world on PS1. His glory years were on NES and SNES not even making Mega-Man a likely candidate for this game at all. If anything, Mega-Man should be in Super Smash Brothers by now.
It's very relevant of where Mega-Man started and was popular where he started. Not to mention Lightning is more of a PS All-Star than Mega-Man ever will be.
It's also relevant where Capcom wants him to go, more so than ANY other factor. I agree that he will probably end up in SSB4, but that comes down to what Capcom wants, and if Nintendo will agree to it. I mean, Snake was popular on PlayStation, but where did he end up? And why was that, exactly?
As for the last part of your comment, Mega Man will never be a "Smash Brother," nor will the LARGE majority of the SSB characters. The title "PlayStation All-Stars" is wholly irrelevant to who is selected for the game.