Well i ask the same question to a Marketing guy from HD au.
and they are 2 main reasons :
1. AU have a restriction for learner drivers ( to have a full license take 3 years)
We can only DRIVE UNDER 640 CC bikes, so they were loosing market with Japanese, English and Italians bikes.
the idea was to bring the 500 first to get that market.
2. PRICE: as you maybe imagine Australia is quite expensive, the 500 in America is around US$ 6500 or so in here is around US$ 8500. So the 750 will sit in an awkward place between the 500 and the IRON 830. even though, he told me than they will bring it, eventually.
Ok this is what he told me, so if kind of a gossip

if any one have more accurate information, please feel free to share.