As Zebo pointed out, this is largely because of the loss of wealth-producing jobs and the increased labor supply. When we increase the supply of labor, both by immigration and by allowing unfettered competition from abroad, we downgrade the value of that labor. The main effect of this has been on unskilled and low education labor, especially since so much immigration has been illegal immigration, but we're starting to see a squeeze on skilled labor and white collar work as well, because as the work is increasingly being done by other countries, that work's requirements for engineering, accounting, and management drop considerably. As people in low wage countries like China learn how to make the products we demand, they will form their own companies to manufacture competing products at lower prices (because of the lower overhead, lower regulatory costs, and lower taxes with the same cheap labor American companies' foreign factories enjoy.) As these start-ups increase market share, we see further drop in domestic demand for engineering, accounting, and management from the shrinking American companies.
And personally, I say screw 'em. If an American company produces its products in the USA, I'll pay more, sometime considerably more, to use their products. But if an American company moves its factories to China and its support to India, screw 'em. I'll buy a foreign competitor in preference, especially if that competitor's products are made in its home country.