My guess, having moved back here, is because there's nothing for the locals. They're almost forced to move away from the area, meaning they take their council taxes etc with them. Any accommodation that is cheap is where the government, in some cases literally, dumps the immigrants. So all the cheap accommodation is no longer available for the young adults trying to get a foot on the housing ladder, meaning they can't afford to move out. Also, and I know this sounds really cold and it is, but old people living longer so there's less inheritance and no properties being passed down.
That's just 1 example. The government's of the last 50 years have fucked the North into oblivion. Like I said, it's all money.