Because a guest would not be able to evaluate the host in ALL categories, and thus it would give the guest a chance to leave an unfair review, @Jessica547.
Your question is not clear to me. Did you cancel during your stay or after check-in ? If so, you should be able to leave a review.
If you have not been or stayed, you cannot leave a review as it is impossible to review a property you have never set foot in or a host you have never met.