Why Filipino-Japanese assimilated rapidly?

I don't really get why i never had earned a Japanese surename since birth or dad simply didn't cared of using that surename. Or maybe, this explains it well.

Early as the 12th century there are either traders or populations itself, and the Tokugawa expulsions in the 16th centuries for Christian Japanese... Oh wait, during the war, a lot of the Japanese-mestizos who are native before World War II were indiscriminately killed, forcing the living ones to change to Filipino names and deny even a bit of their Japanese heritage.

That pattern of assimilation irritates me. As a Sansei, i am slowly discovering my roots and tries to learn Japanese since i have a Japanese born Girlfriend and pregnant with my first, as i will name it Masahiko Đorđe if a boy and Ajko Jefimija if it is a girl. I may call both names because i have a strong leanings on both Japanese and Serbian cultures.

But choosing a Filipino name is not an option. Homestly. Unlike the Brazilian-Japanese or the Japs in America that have either maintained their cultural identity despite all the wartime propagandas thrown against them, the Filipino Nikkeijins simply assimilated unlike the Filipino-Chinese that won't even make a full-scale assimilation to the Filipino culture.

Not for racism or pride but i lament the negative effects of assimilation and the consequences of my family or the group i am born from that never cared about their roots.