smallish population (mid 50s globally)
less depth of big enough companies to sponsor competitive purses.
A much different structure than the USPGA tour which can leverage larger companies and the non-profit status to help generate huge deals.
A long time ago, the television rights were much different. In the US we were selling tv rights, back when I played Australia in early mid 90's, the tour (or tournaments) was (I was told) basically were buying their television time. That was forever ago though, I'm really not certain what happens anymore.
But the structure of the tour and the economic strength of the US PGA tour sponsors started to create an enormous gap in purses.
ANd even points flying in business class from the West Coast, it was a very difficult trip to make and feel competitive when you arrived, but also when I returned home to play.
Without a doubt, private or first class have to be easier, but that is a big assed trip!
When I played, the AusTour golf courses IMO were light years better than my early years on the PGA Tour in architecture quality, but also playing conditions, it was no contest.