Because that's what the game developers decided to do. Pokemon Black & White is 100% new Pokemon, not linked to older ones.
Actually, there are LOADS of continuations:
Pichu, Cleffa, Igglybuff, Tyrogue, Happiny, Mime Jr, Smoochum, Elekid, Magby, Munchlax, Azurill, Bonsly, Wynaut, Mantyke, Budew, Chingling are 'prevolutions'.
Bellossom, Politoed, Slowking, Hitmontop, Gallade, Froslass are branched evolutions.
Espeon, Umbreon, Leafeon, Glaceon are new branches for Eevee.
Crobat, Magnezone, Steelix, Lickilicky, Rhyperior, Tangrowth, Kingdra, Porygon2, Porygon-Z, Togekiss, Yanmega, Honchkrow, Gliscor, Weavile, Mamoswine, Probopass, Roserade, Dusknoir are continuations of previous generations.