Here's why:
* Common nouns refer to general things or people, not specific ones. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns are specific names of people, places, or things and are always capitalized.
Examples:
* Common noun: the coast, a coast
* Proper noun: The Pacific Coast, the West Coast, the Gulf Coast
While you might see "Coast" capitalized as part of a specific name (e.g., the West Coast), it's not capitalized on its own.