Because this needs to happen. Stay Puft is all "come at me bro"
Talking to my brother, I was all for Godzilla. He has teeth, claws, fire breath, of course he'd win. To which my brother pointed out, StayPuft is a ghost, sure the Ghost Busters killed him with a beam, but it was a crossed proton stream. Proton streams being calibrated to catch ghosts, thus the ghost was trapped and that's why stay puft exploded.
Since Godzilla doesnt have ghost trapping powers, every claw, slash or bite taken would potentially just immediately grow back. So all though StayPuft wouldn't win a fight, Gozilla would most likely eat himself into a sugar coma, and StayPuft would just get up and walk away.
So I think in the end StayPuft would win. What do you think? Be prepared to justify your answer!