Here's why:
* Common nouns refer to general categories of people, places, or things. They are not specific individuals or locations.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: musician
* Proper noun: Beyoncé, The Beatles, John Legend
"Musicians" is a general term for anyone who plays music. It doesn't refer to a specific person or group.