As a result of their world cup glories. But you don't think those superstars actually playing in front of a crowd would have a similar effect? Look at the reception the Aussies get in PNG every September.
Nothing to do with World Cup glories, a lot to do with the simple fact that there are far more potential opportunities to earn (much needed) money, playing at even quite low levels in Australia, so there is a powerful incentive to play rugby league ahead of rugby union, unless you are in the disciplined services, or some other secure job. Unemployment is pretty bad.
60% of Tonga's Gross National Product is overseas remittances. If a normal family does not have one member working off shore, they are in deep doo doo.
The last time I was in Tonga, their two biggest grounds could hold maybe 8,000 each, at a pinch. To fill them, think maybe $5 per head, maximum. Even that might be too much.