Licensing and copyright issues, most likely have something to do with it. If the developer is not licensed to sell the app in the U.S. App Store, then it isn't available here. It works the other way around. There are apps in the U.S that can't be purchased in other countries. Some U.S. TV network apps for instance may not be available in other countries.
Whoever holds the copyright to it in the USA hasn't given Apple permission to make it available there. Contact the application's creator.