As with any economic question, it's usually not due to one reason. The combination of energy price and workforce age comes into play. But the biggest trigger of all was the trade with China. That put a serious pressure to job competition, and led to significant wage stagnation in the US. On the other hand the 1% benefited a lot from the trade.
Total US trade with China was vanishingly small well into the 1980s, and didn't start to become significant relative to total US trade until the mid-to-late 90s[1].
I think there's a better argument for trade at large, since ~1970 was a turning point for many _other_ Asian countries, in particular Japan, Taiwan, South Korea, Singapore, etc. There wasn't a clear, sharp line, though.
Probably a little of the other countries, followed predominantly by China, and that the common denominator was unrestricted free trade with poorer countries. As the other countries liberalized and conditions for workers improved they became less valuable places to offshore production to. Mexico and especially China remain countries of cheap labor due to political conditions and thus the high deficits.
Across the US political spectrum free trade has been embraced by economists and politicians alike, all with the idea that capitalism will force liberalism in places like China. And yet, here we are 50 years later and the CCP is as strong as ever. Instead of a growing middle class resulting in better working conditions, it resulted in an insulated and complacent privileged class who either can't or won't demand better working conditions for the less privileged, for fear that they or their families will become state enemies. China's currency manipulation and curbing of worker's rights ensures industrial equivalents in the US are incapable of competing.
So many economists have rationalized the benefits of free trade as being that the country who can make things most efficiently and with the best quality will succeed, but they ignore what goes into "efficiency". But worker safety protections (especially from litigation risk), higher wages, and better time off are all significant sources of inefficiency, from a production standpoint.
The market devotee acknowledges that the market solution to worker protections is unionization and job-shopping, but it's nearly guaranteed to put enough pressure on corporations to offshore as soon as possible. Unions help wages for those jobs which stay, but guarantee there aren't that many left. Free trade with places like China and Mexico is stacking the deck against American labor.
Conservatives have long used unions as a scapegoat for this problem, but it doesn't explain why it's more often the case that a corporation offshores operations instead of moving operations to a state without government mandated unions. There is some truth to their argument, but I don't think they understand that basic safety protections, natural market forces, and potential litigation all contribute to a significantly more costly American worker than one abroad, even if they aren't in a union.
I think that one of the central reasons this has been happening is that the American ruling class has been insulated in their ivory towers and Ivy League schools for too long. They're genuinely more concerned with "saving the world" than creating a cohesive, prosperous society. It explains why so many lives and money is used to intervene in countries on the other side of the planet, why immigration from 3rd world countries is heavily promoted despite fierce resistance from the lower class, and why they have all but abandoned the idea of low-skilled labor becoming more valuable.
As with any economic question, it's usually not due to one reason. The combination of energy price and workforce age comes into play. But the biggest trigger of all was the trade with China. That put a serious pressure to job competition, and led to significant wage stagnation in the US. On the other hand the 1% benefited a lot from the trade.
https://en.m.wikipedia.org/wiki/China%E2%80%93United_States_...