Because you can hide Zoro Killing Kaido in the news right?
If Zoro kills Kaido in front of everyone it would be spread he did it not Luffy and we know that ain't going to happen as Luffy is going to be the one to get credit beating Kaido.
Face it from a narrative point of view it makes no sense.
Either Zoro walks out of Wano recognised as the killer of Kaido and it is in all the papers outshining Luffy which has never happened or Luffy defeats Kaido and he is recognised as the one who brought down Kaido.
Both things can't happen so which one makes more logical sense with everything we know about One Piece.