Here's why:
* He is the only major character to remain alive at the play's end.
* He is the closest friend to Hamlet. He is entrusted with the task of telling the tragic story of Hamlet's life and death.
* The play ends with Horatio's famous lines: "And let me speak to the yet unknowing world/ How these things came about."
While other minor characters like the gravediggers survive, Horatio is the central character who survives the carnage and becomes the narrator of the events.