Here's why:
* Common nouns refer to general things or people (e.g., shop, cat, teacher).
* Proper nouns refer to specific people, places, or things (e.g., Amazon, New York City, John).
"Shop" is a general term for a place where goods are sold. It doesn't refer to a specific place. If you were talking about a specific shop, like "The Corner Store," then "The Corner Store" would be a proper noun.