How much time, effort and money have the RFL spent on expansion clubs over the years, plonking sides in metropolitan or Southern areas with big football clubs?
They need to learn to grow organically; if you can't get a decent club in somewhere like Preston, a few miles from the likes of Wigan and Saints, then what chance in these far flung, isolated places?
Sheffield was the logical progression geographically, just outside the established areas but you also can't ignore the dominance of football in the UK. There is a reason places like St Helens, Warrington and Wigan do comparatively well - decent population sizes without big football teams - see also Wigan's drop in gates as soon as their football team grew in stature. Leeds are the one anomaly but their football team has largely underperformed for decades. As much as RL supporters don't like to admit it, people simply prefer their town/city to be on the big national football stage.
You need firm up support in, lets say West Cumbria, and then you try and get the people of Carlisle interested, but not by pushing them to go full time and into SL but by helping the amateur grass roots game build foundations. Once they are fully established with a base of decent local talent, you can then look to go semi-pro. The RFL are always looking for the miraculous silver bullet that catapults Newcastle, Cardiff, Birmingham or London into the top echelons of the game.