There's no point in expanding if there's no money in it, because if it's not independently sustainable then sooner or later the club is going to fall over, and if the club falls over then all you have achieved is a waste of every bodies time and money.
So no matter how much they might deserve a team and how obsessed PNG is with RL, the reality is that, on average, they can't afford the product, and because of that placing a club in PNG makes no sense from the NRL's perspective.
Despite your obvious personal bias against Adelaide, the fact of the matter is that in their city (BTW by every metric it's a city and not a large country town) of 1.4 million, people have more disposable income on average, there are almost certainly more valuable sponsorship and corporate opportunities, and it is more valuable to local broadcasters than the whole 8.2 million population of PNG put together, and in reality an NRL team in PNG would be based in and mainly reliant on Port Moresby as it's target market.
So in other words Adelaide can afford the product and it's inevitable that they'll get a team, and by rights they should get a team long before quite a few other places that are mooted for expansion.