Here's why:
* Common nouns refer to general things or people, not specific ones.
* "Floor" is a general term for the surface of a room that you walk on.
Examples:
* "I need to sweep the floor." (general floor, not a specific one)
* "The floor is made of wood." (general floor, not a specific one)
Contrast this with a proper noun:
* "I'm going to the Empire State Building." (specific building)
* "I live in New York City." (specific city)