Why are middle class wages not increasing with the economy?

Kevin Drum has what seems to me to be a rather strong argument. He starts with the differences in the wages for average players before and after WW II, and shows what the key difference has been in the last few decades. He then goes on to show how this analogy applies to the general American economy.

Why has the income distribution in the U.S. changed from rather broadly based to instead more relatively poor workers and a few much more highly paid already wealthy individuals? I find his argument quite compelling. Go look at it.