The history of the United States is very short. Why has it become the third largest tourism power in the world?

This depends on many aspects. The biggest aspect should be that after World War II, the economic and technological progress of the United States led to the rapid and steady development of tourism. Moreover, World War II had little impact on the United States, but it was different for other major countries in the world. After being basically involved in the turmoil of World War II and being severely impacted, the United States jumped up and gradually became a major tourist country.