You wouldn’t think that Japanese could assimilate if you were in a predominantly Japanese area of Bangkok or Seoul, and people said exactly the same thing about first generation Japanese in Hawaii, California and Brazil, e.g. “Oliveira Viana, a Brazilian jurist, historian and sociologist described the Japanese immigrants as follows: ‘They (Japanese) are like sulfur: insoluble’.”
Now, however, 61% of great-grandchildren of Japanese immigrants in Brazil have at least some non-Japanese blood, 60% of Japanese-Brazilians are Roman Catholics (only 25% being adherents of a Japanese religion), and the third generation, however, are most likely monolingual in Portuguese. Similar things are true in other countries, for example church going in Japanese immigrants and their descendants in Hawaii and California being much higher than continued belief in Buddhism, let alone Shinto.
I put both the early and more recent histories down the Japanese desire to blend in with society, because in the early days for most people that society would have been just their immigrant community but sooner or later the society to blend in with would be seen (consciously or unconsciously) as the local community.
Most info above from Wikipedia, but all speculation entirely my own.