I suspect too it's a function of the demographic of the crew being recruited. Given the high number of very experienced pilots applying to be S/Os, it's no surprise that the majority of those getting in are often early 30's with lots of time in either jets or turbo props as other long time F/Os or even Captains. Thus they've done 'the hard yards' (so to speak), have established families, with husbands or wives who established in their own careers. These people are simply more competitive than your 20-24 year old Caravan driver from Broome and it's no surprise the recruitment system jumps at them.
Of course having established families in established locations they're not interested in uprooting everything and heading off to Perth to get an F/O slot. The 22 year old Caravan driver who we overlooked as 'not competitive' and instead recommended to Qlink to fly the Dash 8 would have jumped at the chance for a 737 F/O slot after only a couple of months as a S/O. The sad thing is I know of a couple of very experienced people who would have jumped at a 737 F/O slot in PER and would be great addition to mainline. Sadly their 'customer focus' was adjudged not good enough.
It's ultimately supply and demand. We have enough pilots but the supply says 'we don't want to move to PER'.