>>12679376Because it still has some "normal" characteristics, specially as Wigglytuff, they are clearly mammal and look a bit like a cat or a rabbit. They're also rather versatile. I can't seem focusing too much on just fairy moves, they will probably keep a lot of normal moves too.
I mean, I would have made it pure fairy, but I think that's their reasoning for Normal/Fairy. Clefairy and co are much more strange and undefined, and fairy-like.
I doubt they have some kind of rule not allowing types to be changed, only added.