Some of the truest words I've heard in this topic.Marth fans too intimidated by Marth's manliness shyed away and picked Roy as an alternative.
Seriously, people played Roy. While Marth was better, Roy was still favored by many.
Mewtwo sucking in Melee should not merit it not being in Brawl. Kirby was one of the worst in Melee. What they did to Kirby in Brawl (making him whupass) they could have done to Mewtwo. Mewtwo filled a niché for Brawl (the Pokémon antagonist). No one saw Mewtwo being dropped. Even with Lucario's inclusion, no one though Mewtwo would get booted.
What people fail to realize is that yes, Roy WAS a liked AND used character. Nowadays, Captain Falcon is considered bottom tier... Does that prevent people from playing as him? Of course not!
And yes, Mewtwo filled as the Pokemon antagonist. Now, had his legacy been carried on in another equally qualified Legendary (Deoxys or Darkrai, for example), then I know people wouldn't be complaining as much... but knowing that such a highly popular "evil" Pokemon got the boot in favor of time... Makes about as much sense as to why someone like King K. Rool isn't in yet.