Here's why:
* Proper nouns are specific names of people, places, or things (e.g., John, London, Eiffel Tower).
* Common nouns are general names for people, places, or things (e.g., man, city, building).
"Coffee table" is a common noun because it refers to a general category of furniture.
You could, however, make it a proper noun if you were talking about a specific, named coffee table:
* "I bought the 'Oakhaven' coffee table from the furniture store." In this case, "Oakhaven" is a proper noun because it's a specific name for the coffee table.