I've written before about how Asians tend to overassimilate, since their ancestral cultures emphasize conformity. But of all the Asian ethnic groups in the United States, Japanese Americans are the ones who have assimilated the most. They have the greatest percentage of outmarriage of all the Asian American ethnic groups, and this is true for both men and women. Japanese Americans have married primarily white and Chinese Americans.
There are a number of reasons why their community is disappearing. Although the JA community has been in the US for a hundred years, it hasn't had a constant influx of immigrants, like the Chinese American or Filipino American communities. As a result, most Japanese Americans are third, fourth and fifth generation.
But a big factor in the assimilation of the JA community was, of course, World War II and the internment camps. Being marked as an enemy based on your eye shape had profound effects on the psyches of many Japanese Americans that have lasted generations later.
Prior to World War II, there was a huge Japanese American community on the West Coast. The relocation had removed this community, and when the war was over, the JA community dissipated across the United States. Very few returned to the West Coast.
In an effort to mitigate post-war animosity that would result when Japanese Americans were reintroduced back into mainstream society, the War Relocation Authority told the JA community three things:
1) Don't go back to the West Coast to live.
2) Don't cluster together.
3) Assimilate as much as possible and don't call attention to your Japanese heritage.
The trauma of internment had stripped the JA community of cultural pride and knowledge. When you don't have a proud cultural heritage to give you strength and perspective, then you're more prone to being influenced by the greater society, who may not have your needs in mind.