Why Can't Some Pokemon Evolve

Since Pokemon FireRed was released in the 3rd Generation of Pokemon, certain Pokemon Species originally released in the 1st Generation had since received an additional Evolution. One example of this is Golbat, that has the ability to evolve into Crobat (which was released in the 2nd Generation). However, to begin your journey in this game, you will be given the Kanto Pokedex and not the National Pokedex. Because of this, Pokemon in the Kanto Pokedex that received a later evolution will not be able to evolve into said evolution until you receive the National Pokedex in the Post Game. That means Pokemon like Golbat will be prevented from evolving, even if all the parameters are met to do so, if their evolution isn't found in the Kanto Pokedex until the National Pokedex is obtained.