Here's why:
* Common nouns refer to general things or people, not specific ones.
* Proper nouns are specific names of people, places, or things.
"Drove" is a general term for a group of animals, like a "herd" or a "flock." It doesn't refer to any particular group.
Example:
* "The drove of cattle slowly moved across the field." (Common noun)
* "The Texas Longhorns are a famous breed of cattle." (Proper noun)