Here's why:
* Possessive nouns show ownership. They typically use an apostrophe and 's' (e.g., the dog's bone, John's car).
* The sentence "The furniture was simple" is simply a descriptive sentence. It tells us about the furniture, but it doesn't indicate who owns it.
To make it a possessive noun sentence, you could rewrite it like this:
* The store's furniture was simple. (The store owns the furniture.)
* The house's furniture was simple. (The house contains the furniture.)