So what was it that made unions in these now low-paying jobs vanish? What enabled companies to say "we're going to drastically drop wages" and made unions unvia

So what was it that made unions in these now low-paying jobs vanish? What enabled companies to say "we're going to drastically drop wages" and made unions unviable? How did America, seemingly within the span of years, end up with a tremendous number of jobs that "no American wanted to do"--jobs that Americans did and earned living wages for, for many, many years?