Here's why:
* Proper nouns are specific names of people, places, or things. They are capitalized. For example:
* John Smith (person)
* New York City (place)
* Amazon (company)
* Common nouns are general names for people, places, or things. They are not capitalized. For example:
* person
* city
* company
"Hardware store" is a general term for a type of store. It's not the specific name of a particular store. If you're talking about a specific hardware store, like Ace Hardware, then that would be a proper noun.