Here's why:
* Common nouns refer to general things or people, not specific ones.
* "Bag" is a general term for a container made of flexible material. It doesn't refer to any particular bag.
Examples of common nouns:
* Dog
* Car
* Tree
* Table
Examples of proper nouns:
* Rover (a specific dog)
* Ford Mustang (a specific car)
* Oak Tree (a specific tree)
* The Dining Table (a specific table)