Because neither of those things conform to the WA policy of having a week's worth of quarantine for arrivals from Victoria. The reason it worked before was because WA were allowing teams to quarantine (a softer quarantine) in other states for a week before flying in to Perth for the game. NSW was the only state Vic teams could go to in order to have the softer quarantine but the breakout there over the past week has changed the way WA deals with NSW arrivals, so that was no longer an option.

We can argue all we want about the WA government and their standards but whilst it is what it is, the only options were a neutral venue (which WCE was never going for), or postponing the fixture, which I presume the AFL weren't going to accept.