Here's why:
* Proper nouns are specific names of people, places, or things. They are capitalized. Examples: John, New York City, the Eiffel Tower.
* Common nouns are general names of people, places, or things. They are not capitalized unless they are at the beginning of a sentence. Examples: man, city, tower.
"Marsh" is a common noun, referring to a general type of wetland.
However, if you are talking about a specific marsh, then you would use a proper noun to identify it. For example:
* The Great Dismal Swamp is a specific marsh.
* Everglades National Park contains a large marsh.
Let me know if you have any other questions about proper nouns!