There’s an interesting theory floated on a Harvard Law blog.
Smart people used to stay close to home because it was too expensive to move away. You’d only be able to communicate in emergencies, and maybe you could take the time to travel home once a year. This resulted in every town having its share of intelligent people. These people would take on leadership roles within the town, as principal of the school, chief of police, etc.
But now with the ease of travel and free communication, the smart people from towns across America are leaving the hometown and going to live in University towns and cities. Leaving the rest of the towns dumber than ever.