I dont really understand either , I mean , I dont really care wether they are kept in the same order upon evolution , what I really care about is why the fck does it matter if it's Fire/Dark or Dark/Fire(Im not talking about evo's here)you get the double STAB anyway ~_~
This is the typical competitive player response, lol. A Pokemon's primary typing reflects its main characteristics and properties. When I see Gengar I think of a Ghost first, which is why its primary typing is Ghost (not because I personally think of a Ghost, but because Gamefreak had that design in mind). Flygon is primarily Ground, while Garchomp is primarily Dragon. These help create flavor, along with Pokedex entries and such.