Over time, the trend has been for the US to develop transportation and communications networks that have become ever more elaborate and extensive. This has allowed the country to have an economy with large companies that take advantage of regional specialization.
The transportation system makes it possible for goods to be brought from any point in the country to any other point. This allows certain regions to specialize in certain things because the products from those regions can be brought anywhere.
Communications have allowed for larger companies to develop because they can easily stay in touch with suppliers and customers around the country.
Today, the communications (internet) and transportation (shipping, trucks, and trains) has actually helped to create the economy we now have with so many imports coming from abroad.