Agree with #3. I'd like to expand on the third reason, ancestry and culture. This emphasis on maintaining the purity of a culture necessarily causes that culture to turn inward and close ranks, so to speak. Japan has clearly decided it doesn't need or want any help or interference--or diluting--from other cultures. There are positive aspects to that choice, of course, such as a common heritage and national pride. The negatives are isolation and, as you mention, a resistance to change. It appears to be a conscious decision, one Japan has found effective for centuries.
There are several reasons for Japan's uniformity and lack of assimilation. First, because it is made up of a group of islands, Japan has found it easier to isolate itself from unwanted influences. Secondly, in almost all cases, Japan has been the invader instead of vice versa. While Japan went into Manchuria, Korea, and later into numerous other locations during World War II, Japan itself was not occupied. Yes, the U.S. bombed Japan, and yes, Russia went into Japan at the war's end, but neither country actually took over the country. The Russians are now gone, and American troops are still regularly stationed in Japan, but not at the significant levels that they are in Korea.
Finally, Japan's culture places great signficance on tradition and ancestry. Intermarriage (even with other Asian people) is still frowned upon, and the Japanese have proved for decades that they are able to develop cutting-edge technology while still abiding by traditional values and centuries-old beliefs.
we often heard that most country do open to some changes in term of their culture but, why japan resist to change?