My answer: because of lack of social impetus to get educated. I bet if the stories of success as a consequence of education, and the greatness of science were commonplace on, say, Fox News (or any news), society as we know it would be different.
Remember: it is not convenient for the powers that be to have an educated society. What corporations/government want is obedience: buy and agree with what you're told. How do you expect Coca-Cola to promote education if there's a risk of consumers learning about the wonders of HFCS?
Also, US is just about the only country that I've visited that uses the term 'nerd.' Why do you think that is?