Why do so many people seem to think citizens must be represented in Congress by others of the same ethnicity?

Affirmative Action mindset. It makes race much more important than qualifications. But we have a black president in a majority white nation. How can this be? Since it is considered inappropriate to have a white police chief in a predominately black city? If you didn't read about it a while back look it up. If the article hasn't been taken down due to 'racism'.