Automation is the main reason, of course, along with the application of new ways of measuring output and effectively structuring work teams. And as the jobs move from simple assembly operations to overseeing the machines that perform those operations, manufacturing jobs require more than a high school degree. Especially given that modern public high schools are more like warehouses for the delivery of social welfare services than they are educational institutions. But that's a matter for another blog.
The brief point is that you can't walk out of high school now and into a job where an employer will train you to be productive over the course of a multi-decade career. Instead, you have to have some specific skills in math, science, management, and maybe more to even qualify for entry level positions.