There are less traditional families (parents + children ) these days and more single households compared to the 60's. Thus despite the marked increase in individual personal income, the median houshold income has increased only modestly.
Put simply, the average Joe is earning significantly more than previously but he/she chooses to stay single:)
From : http://en.wikipedia.org/wiki/Household_income_in_the_United_States
Since 1967, the median household income in the United States has risen modestly, fluctuating several times. Even though personal income has risen substantially and 42% of all household now have two income earners, the median household income has increased only slightly. According to the US Census Bureau, this paradoxial set of trends is due to the changing structure of American households. For example, while the proportion of wives working year-round in married couple households with children has increased fron 17% in 1967 to 39% in 1996, the proportion of such households among the general population has decreased. Thus, while married couple households with children are the most economically prosperous type of household in the United, their share of the population has been dwindeling in the United States. In 1969, more than 40% of all households consisted of a married couple with children. By 1996 only a rough quater of US households consisted of married couples with children. As a result of these changing household demographics, median household income rose only slighly despite an ever increasing female labor force and a considerable increase in the percentage of college graduates.[23]
"From 1969 to 1996, median household income rose a very modest 6.3 percent in constant dollars... The 1969 to 1996 stagnation in median household income may, in fact, be largely a reflection of changes in the size and composition of households rather than a reflection of a stagnating economy."- John McNeil, US Census Bureau
Interesting numbers, thanks!