Here's why:
* Pronouns stand in for nouns. They refer to people, places, or things that have already been mentioned or are understood in the context.
* Common nouns are general names for people, places, or things (e.g., dog, city, book).
Example:
"The students arrived late. They were tired from their journey."
In this sentence, "they" replaces the noun "students" and is a pronoun.