What is the reason why one of the two couples got a U.S. tourist visa?

Personal reasons. The couple's decision to apply for a tourist visa to the United States was due to personal reasons, because the visa officer believed that one of them was suitable to go to the United States. The United States is located in the center of North America, with an area of ??9.629 million square kilometers (including 9.15896 million square kilometers of land area, 200,000 square kilometers of inland water area, and 170,000 square kilometers of the US-sovereign part of the Great Lakes).