Faries are beings of Light that in popular stories banish darkness and calm violence and dragons. Their natural forest habitat is destroyed by pollution and industrialization. They're helpless to stop their homes from being destroyed by fire, but aren't actually weakened by it.
Alright, thanks. Now I know the background for that decision. But I still do not get it from a playgame angle. Dark already just has the same two strenghts as ghost, so why make it even less powerful by making it weak against fairy?