Here's why it's not a case like nominative, objective, or possessive:
* Nominative: Used for the subject of a sentence (e.g., "I went to the store.")
* Objective: Used for the object of a verb or preposition (e.g., "He gave the book to me.")
* Possessive: Shows ownership or belonging (e.g., "That is my car.")
"Your" falls into the possessive category because it tells us something belongs to "you".