Why are real median household incomes stagnant?

This image shows US real median household income. It seems remarkable for its lack of growth over the last 20 years. This has been the subject of much political debate (around the "1%" and "occupy Wall Street" movements, etc.) but I am interested in more objective economic analysis of this phenomenon. Given that people are better educated and are working with better technology (which should make them more productive), and that the real economy has grown more generally, what economic theories can explain the lack of growth in real median household income?