Why Do More Planes Fly Over The North Pole Than The South Pole?

Scandinavian Airlines, or SAS, recently marked 70 years of pioneering routes to the United States from Europe over the North Pole. Today, many airlines fly over the North Pole (or at least within the Arctic Circle) between Europe and North America, Europe and Asia, and North America and Asia. However, why is there far more air traffic over the North Pole than over the South Pole?