Here's why:
* Common nouns refer to general people, places, or things. They are not capitalized unless they begin a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
"Mountaineer" refers to anyone who climbs mountains. It's not a specific person's name.
Example:
* Common noun: The mountaineer reached the summit.
* Proper noun: Tenzing Norgay was a famous mountaineer.