If you make a big main stream game you need to maximize its audience. Australia has 21,874,900 according to 2009 data, US has 350,000,000 give or take a few million. Not everyone is a gamer....There is a good chance that you have as many gamers in California as in Australia. A game passed on Australia isn't as interesting in the US as the US would be, Australia isn't as interesting as Russia, China, Korea, Europe, the Middle East, or Africa. That's the issue. A game based on Australia isn't as marketable internationally or in bigger nations.
I respect that Australia has a culture and history like everyone else but the reason for a huge investment in a game base on Australian history is total dollars. Now they could invest in DLC that is Australian and have success but Call Of Duty Sydney isn't as appealing on a global scale and Call of Duty North Korea, Iran, or NYC.