Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things.
Since "wardrobe" refers to a general type of storage furniture, it's a common noun.
Here's an example:
* Common noun: I need to organize my wardrobe.
* Proper noun: I bought a new wardrobe from IKEA.
In the first sentence, "wardrobe" refers to any wardrobe. In the second sentence, "IKEA" is a proper noun because it is the specific name of a store.