Until the 70s, the US really had no competition. To compete, you need a stable (legal) system - No one would invest money in a country where it's not far-fetched that someone will overthrow the government, nationalizes your company and throws you in jail.
Oh, and because of geo-politics, one couldn't invest in USSR friendly countries (both from the US side and the other side)
So no one would invest in the USSR, Africa, the Middle East, China or India, Europe and Japan were in shambles. So who built up industry? The US.
After the 60s, different countries started stabilizing. First Japan, then China, now most of the world is actually quite stable, so now the average US worker has to compete against all of India, China, Bangladesh, etc.
You have more supply and the same demand (the world).
And the problem is nothing can really be done about it.