Again, they probably won't even ANNOUNCE a new SSB until at least 2012, at which point I'm thinking we'll have a new FE game.
They were trying to keep Roy in, but were short on time/space. They actually didn't mean to cut characters other than Young Link (replaced by Toon Link), but again, there wasn't enough time, and probably not enough room. They should've made Brawl a 2-disc game: Disc 1 with 1-player modes and co-op modes, and Disc 2 with Multiplayer. That way, they could have a LOT more in the game. But I don't think space was the primary issue, considering how they had to keep pushing back the release date. No, it was probably a bunch of anonymous death-threats saying "DO IT NAO OR I'LL FUCKING KILL YOU".