You will have a better idea than me about the reasons for the lack of Aussies attending games, speaking from a UK perspective I think It's unfair to compare the numbers of RL fans who travel from England with RU followers of the Lions. The working class game point and affordability is not the main issue, it's the lack of numbers that follow RL in general. Football in the UK is predominantly working class, people with similar economic means as RL followers, but you will see tens of thousands of England fans making the trips to major tournaments all over the world, likewise Ireland, Scotland etc. Had the latter two qualified for Russia there would have been 20k from both, easy. RU being a private school game will have wealthier fans yes (generally speaking), but they also have a much bigger fan base to call upon. The Lions have hoards of English, Irish, Scots and Welsh fans who travel, whereas England RL is predominantly fans from Yorkshire and Lancashire. On the BBC radio podcast this week Dave Woods was interviewing groups of English fans in Australia and there was a Yorkshire vs Lancashire banter among them. The major cities in these two counties are not RL areas either, bar Leeds and Hull. Most of the Super League clubs are based in towns.
RL is having to work with a much smaller fan base. Each Super League game averages 8.5k, and there are 6 SL games per week, that's 51k total attendance for the league. Add this to the three lower divisions and it doesn't pass 75k total. The average attendance for a Manchester United home game matches the total weekly number of RL match going fans in England. The percentage of RL fans that attend/travel to games is probably similar to the percentage of football fans that do similar, it's just there are a lot more football fans so the numbers are vastly different. The disparity is even greater when you consider the prices; I pay £25 to go to a St Helens game, I'm paying almost three times that for a Liverpool game at Anfield.
So numbers is a huge factor when it comes to England RL fans, there just isn't enough of us. With the sport not established in Ireland, Scotland or Wales the travelling numbers from those countries will be very small, and those that are at games supporting them are largely expat communities in Australia. Similar with Lebanon, Tonga albeit those were on a much bigger scale.
If RU had the same number of travellers for the 2003 RUWC in Australia as RL has had for this that 2003 tournament would have had plenty of half empty stadiums. England vs Wales quarterfinal in Brisbane had 45k, what would that have got in this tournament? 12 to 15k?
The host nation of a RLWC is handicapped by the lack of fans attending from overseas. I can't think of another major event that has this issue. An Olympics, a FIFA World Cup, those two obviously stand out for the vast influx of people attending from all over the world. The RUWC has a decent number attending from overseas too. RL is probably asking too much of its hosts to buy up all the tickets, especially in the case of Australia where the event has lacked competitiveness for such a long time.