Two main reasons.
Legally it's difficult to get big and not get noticed. Censorship laws here are quite strict. Much of what is available on most adult websites would be refused classification here. Things such as watersports, scat teen looking teens, fisting, sado masochism, amputee porn, sister fantasies, mother daughter fantasies, men dressed as babies, would all be illegal to sell in Australia. Unfortunately from a legal perspective RC material ends up being lumped in with child porn and bestiality which are also RC, quite a serious offence if you happen to cross over the very unclear lines.
Our peak adult industry lobby group, the Eros Foundation recently made a submission on such matters to the Law Reform Commission
http://www.eros.org.au/index.php?opt...67&Ite mid=23
The second reason is that producing porn is very expensive in Australia compared to many other parts of the world. We have high labour costs. If you want great looking talent you had better be prepared to pay for it.