Because you are at capacity now. Don't get me wrong, I'd love to see a smaller divide between N and S, but if you start investing heavily in the N now before it actually has a meaningful impact on people's internal economic migration/commuting patterns it's going to be at least a decade (convince companies to move there, start hiring, create a salary inflation cycle from competition, etc...)

Right now, London salaries are easily 2x to 3x Manchester's for Software devs (yeah I know the average data is only 30% higher but you need to compare the top vs the top, not the aggregated data). As a Junior dev moving from Hertfordshire (which is 30mins from London....) to London meant my salary instantly doubled, while living cost only maybe increased 50% if we are conservative. That's going to take a long time to offset.