Here's why:
* Sirius Black's Escape: The book opens with the news that Sirius Black, a dangerous criminal, has escaped from the supposedly inescapable prison of Azkaban. This sets the stage for the entire story and introduces the central conflict.
* The Threat of Sirius Black: Throughout the book, the fear and danger surrounding Sirius Black dominate the narrative. Everyone believes he is after Harry to kill him, and Harry himself is constantly on edge.
* The Truth About Sirius Black: The climax of the story involves the revelation that Sirius Black is not actually after Harry to kill him, but is actually Harry's godfather and was wrongly imprisoned for the crime of betraying James and Lily Potter.
* The Resolution of the Conflict: The book ends with the capture of Peter Pettigrew, the real betrayer, and the exposure of the truth about Sirius Black, freeing him from Azkaban.
While there are other events in the book, like Harry learning the Patronus Charm and the introduction of Remus Lupin, they are all ultimately connected to the main event of Sirius Black's escape and pursuit of Harry.