Why do people always use the recent games as an excuse for characters not being relevant or don't deserve to be included or restricting the abilities they were able to do before. Mind you I'm not a fan of pairing up Dixie & Diddy but this mindset is what I'll never understand.
I think it was just something many of us noticed with the Brawl newcomers.
Meta Knight - started getting playable roles in Kirby games, starting (I believe) with Kirby: Nightmare in Dream Land
Pit - NES Kid Icarus for the GBA in Japan
Zero Suit Samus - Metroid: Zero Mission
Wario - got a new WarioWare series
Snake - got added as a favor to Sakurai's friend's son
Ike - Fire Emblem: Path of Radiance
Pokémon Trainer Red - Pokémon FireRed & LeafGreen
Diddy Kong - started appearing in Mario sports/party spin-off games while the Donkey Kong Country series was dormant
Sonic - insane fan demand in addition to Mario & Sonic at the Olympic Games
Lucas - Mother 3 (was also planned in Melee)
King Dedede - more Sakurai bias towards his own characters
no, seriously, King Dedede was always the main antagonist of Kirby series
Olimar - new major first-party IP, Pikmin, Pikmin 2
Lucario - Pokémon Diamond & Pearl (and the movie)
R.O.B. - Mario Kart DS
Toon Link - The Legend of Zelda: The Wind Waker
Wolf - more Sakurai bias towards Star Fox 64
Notice that most of the newcomers were relevant in post-Melee/pre-Brawl games.