Why is the first-person point of view effective in telling this story?

Choosing a narrative point of view is one of the most important decisions an author can make when writing a story. Simply stated, a story written in third-person voice will not resemble a story in the first person. First-person point of view puts you inside the mind of a character within the story, so that narrator and character merge together, rendering everything within the story subjective (because the person telling it is tethered to their own personal perspective of the events of the story). This is in sharp contrast to third-person voice, where the narrator is always outside the events of the story.
Poe understood the nuances of first-person narration perfectly and employed them to extraordinary effect. In stories such as "The Cask of Amontillado," he places his readers directly within the disturbed mindset of the character who is telling the story. In this case, we are reading a story in which a person is buried alive, from the perspective of the person who has committed the murder. This means of presentation greatly heightens the tension and horror of the story and creates a disturbing quality that could not have been as effectively achieved were this story told from a less subjective perspective.