One of the things people often seem to overlook in the US is that at about the time the wage stagnation started, we also began the process of roughly doubling the workforce by removing barriers against women entering the labor market. Then, as you say, another labor surplus was generated as the developing world has come into the market.

It's rarely as simple as supply and demand, but there seems to me to be a correlation between stagnant wages and large increases in the labor pool.