There is another force that hasn't been mentioned yet and that is the business community which in Sydney has enormous clout. During the 1960s' Australia's leading business city was Melbourne but there was a gradual shift during the 1970s' to Sydney - life style - weather and such things were perceived to be better and business was actively being enticed to move there and it worked.
Now, the hassles of negotiating Sydney airport and the general lack of infrastructure in a city notorious for its hedonistic lifestyle (i.e. more play and no, we don't want to pay for infrastructure) and the soaring property values are starting to make Melbourne look more attractive. For the well-heeled businessman it even has a nice corporate airport to park the runabout and its closer to the city than the main airport and it has a second airport to keep the riff-raff out of the way for when we have to slum it on a mainline airline.
Ben Sandilands has run an opinion piece on the relative merits of the two cities and so long as the howling unwashed continue to dominate politics Sydney will never get a second airport, something it desperately needs if it is to keep pace with the rest of the world, or even Melbourne.