Its not so much that as more the fact that "Celtic" people never referred to themselves as such at the macro level unlike the Slavs. The term doesn't come into play to lump people together who have an apparent "Celticness" together until much much later. It is a type of exonym rather than an endonym.
It is possible that certain subclades of R1b picked up the Indo-European cultural package from R1a groups around Central/Eastern Europe and then moved West spreading the language and culture to their R1b kin.