Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized.
* Examples: John, Paris, Eiffel Tower
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
* Examples: man, city, building
"Drill sergeant" is a common noun that describes a specific type of person. It's a general term for someone who trains soldiers in military discipline.
However, if you are referring to a specific drill sergeant by name, then that name would be a proper noun. For example: "Sergeant Jones is a tough drill sergeant."