NBA has what 5 games per week for a team (for 30 teams) with 20,000 attendance?
NFL has 1 game per week (for 32 teams) at grounds catering for anywhere between 80,000 and 120,000.
Their population is approximately 15 times the size of ours. If you do the math with their competitive sports attendance factoring in NHL, NFL, NBA, MLB, NCAA basketball and grid-iron.... They come out well ahead per capita than what we do in terms of attendance for live sports, in fact they completely demolish us. They get crowds of 80,000+ attending college football games.
Their population size has little to do with it, they have far greater attendance to sports than we do in general.
What the NRL doesn't get is by moving games they don't create demand, they lessen it. For a one-off cash grab they cheapen the game. We need bigger suburban grounds without a doubt, but every time a game looks like it may sell out you don't annoy the ticket buyers by shifting the venue.