Not that I'm claiming to be an expert, but you simply don't understand how commercial broadcasting licenses and advertising works.
I'm oversimplifying it a lot (otherwise I'd have to write a book just to try and explain it all, and I'm definitely not the person to write that book), but free to air broadcasters are licensed by the government to broadcast in designated markets across the country, and there are only so many licenses for each market.
Obviously there are metro markets and regional markets, and the metro markets are pretty self explanatory (in yellow), but the regional markets represent large disparate populations, not just singular regions/cities.
This creates markets within the broadcasting industry that aren't really natural outside of it. For example, as far as broadcasters and their advertisers are concerned the Sunshine Coast and CQ already have NRL content representing "their" market, the Cowboys.
This also means that outside of the potential of underserved markets where it might be worthwhile to add another NRL product for ratings reasons (which is debatable depending on the market and is a separate discussion), that every 'RL heartland' broadcasting market has at least one NRL team representing it (most more than one), except for Griffith and Central and East which aren't big enough to support teams, and the ratings and advertising gains from having their own teams wouldn't be there to make the venture worthwhile to broadcasters and major advertisers anyway.
In other words, aside from potentially underserved markets, the NRL's heartlands are 'shored up' from the broadcaster and major advertisers point of view.
This post is already a lot longer than I'd hoped so I'm going to be somewhat simplistic again; but to blue chip advertisers the most valuable timeslots to advertise on are primetime on the main channels. Obviously that's because, on average, that is when the most people are watching at any one time.
So, the programs that are most valuable to broadcasters to put into those timeslots are the ones that can attract the highest amount of viewers across as many markets, particularly the metro markets for obvious reasons, as possible, because that maximises ratings across as much of the population as possible, and makes their advertising space during those timeslots as valuable as possible to as many advertisers as possible.
In the case of sports products that means that the products that have teams in as many of those markets that can sustain one as possible are going to have a competitive advantage over the others that don't, as even if it's not the highest rating sports product in all of the markets that it's in it's still way more valuable to broadcasters and advertisers than a product that rates really well in half the markets but not at all in the other half.