A PNG side is a lose-lose for the NRL.
They may not realise it, but they don't really want a PNG side in the league either, at least not yet, and not only is the support offered by the government unsustainable in the long term, but no amount of it will make a PNG side commercially viable. In other words allowing a side from PNG into the comp would not only be the NRL being forced to accept a side they don't really want, but they'd be setting themselves up with a ticking time bomb that they'll be forced to deal with sooner or later.
On the other hand an AFL side in Tasmania is commercially viable, it's just not as profitable as other options, so the AFL has negotiated a position where they give it the best start and maximise it's potential as much as possible or it doesn't happen.