In the 1970s and 1980s, the US economy was much more closed off to the rest of the world. There were fewer tariffs and other trade barriers, and US companies faced less competition from foreign competitors. This made it easier for US workers to earn high wages, even with relatively low levels of education and skills.
Today, the US economy is much more integrated into the global economy. US companies face stiff competition from foreign companies, which often have lower labor costs. This has led to some US jobs being offshored, and it has made it more difficult for US workers to earn high wages.