>>39092500If the dex is to be believed, Ralts only approaches cheerful trainers and both it and Kirlia need a positive trainer around them to grow. Perhaps it's impossible for them to reach Gardevoir stage without a positive outlook on life.
So that would at least explain why the serene Gardevoir is the common type. Tho I've got to imagine its spirit can still be crushed after evolution, which has got to be especially devastating for this type of pokemon.
