The truth is, if we wanted to we would. Australia's only nation-wide sport is cricket, and we are the world number 1's. Soccer is relatively minor, which accounts for its status, but as it grows in popularity the standard is growing too.
Rugby is a sport dominated by two states; New South Wales and Queensland, with decent followings in Western Australia and Victoria. That's why we are always one of the best (don't talk about the World Cup, you didn't win it either!)
Australia's sporting culture, which accounts for our fitness and strength levels, is fantastic and as a result we are just very good at many sports.