Mainly, because when you move to a racially homogenous society, the feeling of blackness or whiteness erodes. In my travels throughout Europe, I rarely ran into a white person who identified themselves by race. Racial identity loses its steam, when there isn't racial variance. That's why I reject the notion of "feeling" like one race or another as being something intrinsic. So, race, in the context of this conversation, definitely functions like ethnicity.