I’m really surprised no one here has mentioned this yet, but a huge factor would have to be globalization and the offshoring of American manufacturing.
It started in the 70’s, with companies like GE and the car manufacturers moving factories to Mexico and later Asia, and with growing supply of imported cheap goods like steel. This really took off in the 80’s and 90’s with deliberate market liberalization and promotion of globalization during the Reagan/Bush and Clinton administrations.
In other words, American workers’ wages were pressured by the extremely low wages of overseas labour.