There is a bit more to it than that.
PNG have had a long season, have done a huge amount of travelling and live away from their families.
They won the game that mattered last Sunday, went home as national heroes (which I doubt the Penrith players are considered), then had to regroup and travel from Port Moresby to Sydney for this.
I'm sure most of the players just want to get back to their families, wives and girlfriends - something they have only done on the two bye rounds since the season started. It is a concept that Sydney based teams would be clueless about, given their whole competition is essentially taking place within a 200km radius. Travelling to a different country every second week and living in camp for the entire season like PNG do must be a huge effort.
That said, the quality of Premiers does change year by year in both comps, Northern Pride were an outstanding team in 2014, Burleigh last year and PNG this year less so.