Here's why:
* Proper nouns are names of specific people, places, or things. They are always capitalized. Examples: John, London, Eiffel Tower.
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence or part of a title. Examples: man, city, building.
"Kitchen" is a common noun because it refers to a general type of room in a house.
However, if you were referring to a specific kitchen with a unique name, like "The Gourmet Kitchen" or "Grandma's Kitchen," then it would be a proper noun and should be capitalized.