Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they are part of a proper noun.
* Proper nouns are specific names of people, places, or things and are always capitalized.
Examples:
* Common noun: I live in a town.
* Proper noun: I live in New York City.
"Town" is a general word for a small urban area. It doesn't refer to a specific place.