Going back to the early 1900's, a majority of average American workers were not highly educated (college degrees were not mandatory), and most worked in factories, or "blue-collar jobs". This continued through the decades until automated machinery took the place of human workers. The more we installed this machinery to do these jobs, the more workers were laid off.
Today, college education is essential for any job. Since nearly all of our production is by machinery, the only people companies look to hire are educated individuals who will work the "white-collar jobs" (office jobs, etc.)