My understanding is it's because of lack of house in the places with highest demand. The article gives the following example: "The economist Paul Cheshire note

My understanding is it's because of lack of house in the places with highest demand. The article gives the following example: "The economist Paul Cheshire notes that between 1980 and 2018 the declining industrial areas of Burnley and Doncaster built 56,340 houses while adding 22,796 new residents. By comparison, the high-wage university towns of Cambridge and Oxford built just 29,340 houses for a combined population growth of 95,079"