I have no expert or inside knowlege, but I do know that MEL applies exactly the same XW/TW wet/dry as SYD, so I seriously doubt it is politically driven or airline driven. Probably closer to the truth would be ICAO standards.
Yes, AIPA did issue such a directive many years ago, but that was a response to political noise sharing (exactly as Keg said). It never had any influence on SYD airport modes to my knowledge.