>>19489644Because Fairy is a type put there to replace Normal type in some pokemon, and act as a secondary type in others, in both cases to try and make them more useful. Celebi is already double typed, and neither of those types is Normal, so no luck.
If we're going with that logic, each and every single cutesy legendary should be fairy type, because they all have that vibe. Not quite as much as Celebi the most of them, but still more than enough.
