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The answer is pretty straightforward. It's the revenue that can be generated.
If you want to find another reason just look at i) the capacities of the stadiums, ii) the quality of the facilties at the stadiums' iii) the public transport and infrastructure.
Most of the Wobblies Tri-nations matches are held in either Brisbane, Sydney or Melbourne. Brisbane boasts one of the best rectangular stadiums in Australia with a capacity of some 52,500. Melbourne has, in addition to the MCG, the Etihad Stadium which has a capacity of over 53,000. This year's first Bledisloe match is to be held at Etihad and the seating capacity can be increased depending upon how the lower tiers are moved to suit a rectangular field of play within the oval stadium. Sydney has the primary venue for the ARU which holds over 80,000.
In all cases the stadiums have very good facilities and are well served by public transport and/or are within easy reach of the CBD in any case.
It took me a while to work it out but I now travel east at least twice each year to watch tri-nations matches. This year it's going to be both Bledisloe's, in Melbourne and Sydney. I haven't been to the Etihad before but I have attended matches at the other stadia plus others such as the SFS. We will never get a top flight tri-nations match (ie Bledisloe) whilst we have nothing better to offer than a decrepit Subi with cr@p facilities and similar infrastructure.