History of Asia

Why did such a small number of US exports go to China and Japan?

The premise of your question is actually not accurate. The United States has significant export relationships with both China and Japan, and they are some of the country's largest trading partners.

In 2021, the United States exported over $156 billion worth of goods and services to China, making China the third-largest export market for the United States (behind Canada and Mexico). Similarly, the United States exported over $76 billion worth of goods and services to Japan, making Japan the fourth-largest export market.