The explanation is that some Pokémon have adapted to the distinctive microclimates of a region, which makes sense, but it can also be seen as a creative means for Game Freak Inc. to retool classic franchise characters with new Pokémon types, and abilities better suited for gameplay for the Pokémon roster for that specific region.
The pre-evolutions of Raichu, Exeggutor, Marowak, Weezing, Mr. Mime, Typhlosion, Samurott, Lilligant, Braviary, Sliggoo, Avalugg, and Decidueye do not themselves have regional forms. Alolan Forms all consist of Generation I Pokémon. Unlike Alolan Forms, there are Galarian, Hisuian, and Paldean Forms of Pokémon from beyond Generation I, and one Paldean Form and several Galarian and Hisuian Forms have an evolution that their regular form does not.