Drugs cost less in other developed countries because those countries impose price controls. Because drug development has a high up-front cost, but low production cost, drug companies are willing to sell to other countries even though selling just to those countries would not allow them to recoup research costs. Therefore, US consumers are paying for the research costs that benefit all countries. Allow US consumers to import drugs from other countries would lower the revenue of drug companies, which could limit the amount of research they can do.
See
http://www.slate.com/id/1005389/ for a good explanation.