Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are the first word of a sentence.
* Furniture refers to a general category of objects used to furnish a home or office. It's not a specific item.
Examples of how "furniture" is used as a common noun:
* "The new apartment came with some basic furniture."
* "We bought a new set of furniture for the living room."
Let me know if you have other words you'd like to classify!