"Most of the workers in those industries were unskilled and when the coal mine/steel mill/factory closed there weren't any new jobs in the area that they were qualified for and they weren't able to re-train for anything else."
It was the lack of new jobs that was the real problem. The old industries closed because of cheaper overseas competition. The early C19th mechanisation of those industries also made old skills obsolete but replaced old jobs with many new ones. The challenge governments face now is to recreate that situation.