Despite the weak U.S. dollar, a boom in international travel around the world hasn't translated into an explosion of foreign tourists to the United States. Explanations range from post-9/11 security headaches and lower airfares elsewhere to poor marketing by the U.S. Whatever the cause, travel industry experts say the U.S. is missing an opportunity to make up for the shortfall in domestic tourism caused by high fuel prices. At Heli USA Airways, one of several operators that whisk visitors on aerial tours of the Las Vegas Strip and nearby Grand Canyon, vice president of marketing and sales John Power said the faltering U.S. economy and competition from other countries are crimping business. "Right now, there's some other worldwide destinations that are taking some of the marketplace," said Power.