Because nationality does matter. The oft-repeated meme that we're all the same doesn't hold to even superficial scrutiny.
Nationality is not arbitrary in the sense that it's a crystallization of centuries of cultural values. People's place of birth predicts their attitudes about a vast number of political and social subjects. This includes, for instance, how they feel about such subjects as:
- democracy
- freedom of religion
- minority & women's rights
- the belief (or not) that certain races are superior to others
- etc...
(Apologies for the multiple edits. It took me a few tries to express myself clearly.)