Why is the nature of fortunatos insult against montresor not explained in detail?

The nature of Fortunato's insult is not explained in detail in Edgar Allan Poe's "The Cask of Amontillado" because the story is a first-person narrative told by Montresor, and he chooses not to disclose the specific insult. This lack of detail adds to the story's ambiguity and creates a sense of mystery and intrigue.