In economics you end up with a reversion to the mean over time. China was in a very bad state though terrible management over the preceding hundreds of years, so catching up was rather simple. US, is coming down from being the only modern state that was not destroyed by WWII.

US lower and middle class benefited from cheap goods, and suffered from continued automation and imports. However, the US produces more goods now than at any point in it's history so you can't really blame things on a lack of manufacturing.

IMO, I would blame 5 primary factors, massive undocumented population suppressing wages, developed economy making growth harder, tax policy's that harm workers directly and indirectly, unions shrinking, and rapid automation continuously disrupting the economy.