To give you a better nicer explanation to why Starmie or Staryu won't be playable is this:
Only the very popular pokemon will make it. Pikachu is the unofficial mascot of Pokemon, that is why he is in all 3 games. Jigglypuff was in the 1st 2 (not confirmed yet) because she was popular in Japan. Staryu & Starmie have a low popularity. This is just part of the tragedy of the Pokemon series. Once a couple of new generations shown up, the older ones are often forgotten. I bet there are people out there who know who Lucario is, but don't know who Starmie is. Which is why i don't get into pokemon anymore. I was loyal to the 1-2 generation And most of those Pokemon were forgotten once 3rd and 4th came. But at least Staryu was a pokeball and trophy in Melee, and Starmie was a Pokeball in the 1st Smash Bros. So i believe the chain will continue and Staryu or Starmie will be in a pokeball. yes Starmie or Staryu will be in brawl, just not playable.
I hope that was a better explanation then Platnum could.