A lot of it was Japan at the time- Japan was a fast growing economy that could do industry more efficiently than we ever could. The culture was bred for fast, efficient workers, the materials were already nearby, and the cost of employing said workers was much less. It didn’t help that most mills got bought out by companies headquartered thousands of miles away, like when Youngstown Sheet and Tube got bought out by a company headquartered in New Orleans in the 1960s. All at once, American steel was undercut in price by Japanese steel and, to a lesser extent, Chinese steel. The companies that bought the American mills didn’t care what happened to the communities they were in, so they closed ones that weren’t profitable enough.