Here's why:
* Nominative Case: This case is used for pronouns that are the subject of a verb.
* Examples:
* Who is coming to the party? ("Who" is the subject of the verb "is coming")
* Who wrote this book? ("Who" is the subject of the verb "wrote")
Important Note: "Who" is used for people. For things, we use "which" (nominative case) or "what" (nominative case).