Here's why:
* Proper nouns are specific names for people, places, or things (e.g., John, London, Amazon). They are always capitalized.
* Pronouns are words that replace nouns (e.g., he, she, it, who, whom). They are not capitalized unless they start a sentence.
"Who" is a pronoun that refers to a person or people. It's used to ask questions or to replace a noun in a sentence. For example:
* Who is coming to the party? (asking a question)
* The person who won the lottery is very lucky. (replacing the noun "person")