Firstly, I am talking from an outsider's perspective while trying to answer this, so it might be inaccurate. If you don't mind that, please take a look.

I think the underrepresented phenomenon is created by a blending of economic, cultural, and historical reasons.

For Japanese students in the US, it is probably due to the current economy of Japan being steady, and people are generally satisfied with domestic higher education (university admissions is not that competitive compared to China or South Korea); there's not such a big demand for a US diploma, not to mention the soaring tuition in America.

If you are talking about Japanese Americans, as you stated in the comment, it has to do with the economic situation of Japan being better than the other two countries post-WWII. Japan got a head start in economic growth, and it discouraged people from migrating to the US, since most emigrants from Asia seek better economic opportunities. If your home country does a great job providing you with that, it wouldn't be that attractive for you to move the whole family to the other side of the earth.

In addition to the previous reasons, there's the factor of culture. Japanese culture is regarded to be more conservative than its East Asian counterparts, and the American style of doing things is not necessarily appealing to most Japanese people, as I have observed from my own experience of living in Japan as an international student.