That, and the total refusal on the part of steel companies to reinvest profits into new innovation or efficiency gains.
US steel companies made some boneheaded moves when it came to union contracts in the 30 years after WWII, but still made a heavy amount of profits well into the 70s. Like so many old line companies, the Steel firms managment was taken over by accountants and slaves to Wall Street, and they refused to invest in new technology that could have kept them ahead of the curve.
Yes, the union contracts would have still been a problem, but in the end, the buck stopped with the managment. They decided to boost dividends and invest money into other ventures such as shopping malls, airlines and even sports teams. An excellent book to read on how corporate managment in the US lost its way is the "Wreck of the Penn Central", that came out in 71.
Another example is how Roger Smith at GM refused to invest profits back into GM itself, and wasted billions unopn billions on Areospace and other unrelated industries such as Hughes Aerospace.
Just rode through Ohio and passed lots of empty steel mills. The mills that made the steel are out of business because it's cheaper to import the steel from Japan (or China).