Black and white was a reboot pretty much. 150 new, being the red and blue of the new series.
X and Y are like Gold and Silver, introducing a new type. I wouldn't be surprised if you can go back to the other region in this. Don't forgot this introduced a new type like gold and silver. Shouldn't there be one more? --- Sever the skull! His mind lies in peace, free from the torment of existence.
While this is by no means a valid reason, I do believe that GameFreak does like the idea of an even number of types. It was even juxtaposed during the trailer right when Fairy was revealed. --- [Este mensaje fue borrado al deseo del dueno]