Here's why:
* Pronouns stand in for nouns. They replace a noun that has already been mentioned. Examples: he, she, it, they, we, you, I, me, him, her, them, us.
* Relative pronouns introduce a relative clause, which provides more information about a noun. They connect the clause to the noun it modifies. Examples: which, who, whom, whose, that.
Example:
* Pronoun: "The dog wagged its tail." (The pronoun "its" replaces the noun "dog.")
* Relative pronoun: "The dog, which was brown, wagged its tail." (The relative pronoun "which" introduces the clause "which was brown" and modifies the noun "dog".)
In summary: While "which" can act like a pronoun in some ways, its function is to connect clauses and modify nouns, making it a relative pronoun.