Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are at the beginning of a sentence or part of a title.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: The race was exciting to watch.
* Proper noun: The Boston Marathon is a famous race.
It's important to remember that "race" is a social construct and not a biological reality. While it's used to categorize people, there's more genetic variation within racial groups than between them.