You might as well ask why the Japanese don't import clothes made in the US, why they restrict imports of US fruits, US beef, just about anything. The Japanese have always maintained non-tariff barriers to imports. They used these barrier to foster a Japanese domestic car industry.
Of course in most cases American cars are not suited to the Japanese market. American manufacters don't attempt to tailor models to Japanese markets. The Japanese on the other hand do tailor models to the US market. In the past US automakers have generally set-up foreign sbusiduaries to assemble and sell vehicles in foreign markets. The Japanese government discouraged this in Japan.
Even today realtively few foreign built cars are imported in Japan. Less than 300,000 in recent years. In recent years these have mostly been high end European cars, but some US cars are still imported into Japan. At the peak in 1996, around 439,000 cars were imported into Japan - over half of these were US brands.
Ed White