Here's why:
* Common nouns are general names for people, places, things, or ideas. They are not capitalized unless they start a sentence.
* Proper nouns are specific names for people, places, things, or ideas. They are always capitalized.
Examples:
* Common noun: market, city, dog, happiness
* Proper noun: Walmart, New York City, Fido, The Declaration of Independence
"Market" is a general term for a place where goods are bought and sold. It doesn't refer to a specific market. If you were talking about a specific market, like "Farmers Market," then "Farmers Market" would be a proper noun.