The collapse of the US auto industry had a lot to do with that. As Americans became disgusted with the poor quality of American cars and started buying foreign cars in increasing numbers, there went the jobs.
The auto decline has a very long history, and management, unions, and government all share part of the blame. And some must be directed at the “Golden Accident” that left American manufacturing without ANY competition for almost 20 years. After WW II, neither Germany nor Japan could effectively compete with us until after about 1960.
This in turn led management in Detroit to believe they could grant any concession to unions, no matter how uncompetitive; caused management (as David Halberstam says in “The Reckoning”) to think they could never be wrong, and to cease innovating; led unions to ask for (and receive) the moon; then gubment increased taxes, imposed regulations, fiddled with environmental controls; created the Consumer Product Safety Commission, and so on.