Pokemon
¤ Red and Blue
¤ Red and Green (Japanese)
¤ Blue Version (Japanese)
¤ Yellow Version
¤ Gold and Silver
¤ Crystal Version
¤ Ruby and Sapphire
¤ Emerald Version
¤ FireRed & LeafGreen
¤ Diamond and Pearl
¤ Platinum Version

Universal Information
¤ What is this section?
¤ Breeding Pokemon
¤ Elemental Types
¤ Pokerus Infection
¤ Shiny Pokemon

Affiliates
¤ Amity Square
¤ Dark Eon Hideout
¤ Eevee's Headquarters
¤ Jenviousity
¤ Lugia's Island
¤ Perfect Absolution
¤ Pokemon Inferno
¤ Pokeshore
¤ PokeTech
¤ Pokeuniverse
¤ Pokemon Valley
¤ Sapphire Milotic
¤ The Spriting Shack

The answer is simple. Fire and Leaf is suppose to remain as close to the orginal games as possible from start (Pallet Town) to finish (Indigo Plateu). Because those Pokemon's newer evolutions did not exist in Red or Blue, they cannot exist in the first half of the game nor can they exist in the current Pokedex.

So now that you've defeated the Elite 4, those Pokes still won't evolve. There is one thing you must do to get them evolve. If you have over 60 Pokemon, Professor Oak will upgrade your dex. If not...

The first thing to do is to capture / evolve 60 or more Pokemon. When you have done this, speak with Professor Oak. He will then upgrade your Pokedex to the National Dex, which will include post-Red/Blue Pokemon. You can now evolve Pokemon with post-red/blue evolutions.