Supply and demand.If GWS can host at Engie, we can host at KP. What's the difference?
GWS will struggle to fill the Showgrounds Stadium (Engie) for any game including a final except against Sydney, or perhaps a preliminary final against a bigger club such as Collingwood or Carlton. So any final they host where the anticipated crowd would far exceed the capacity of Showgrounds Stadium (>30k) should be moved to a larger stadium within the state, i.e. the SCG or Accor Stadium.
Any final Geelong play will likely exceed an expected crowd of 55k therefore should be played at the MCG to meet demand unless there are circumstances which render the MCG unavailable (i.e. 4 games to be played in Victoria in Week One).