Poe needed something to lure Fortunato underground where Montresor could chain him to the rock wall and leave him to die. Wine was the best lure because Fortunato loved wine and was already drunk when Montresor found him in the street. But what kind of wine? It obviously couldn't be Italian wine because there was an abundance of that at all times, and especially during the "supreme madness" of the carnival. Montresor couldn't say that it was French wine because he was French himself and was a connoisseur of wine. He twice offers Fortunato French wines when they are down in the catacombs: Medoc and De Grave. The only other alternative was a Spanish wine. The Spanish export a lot of sherry, and the finest sherry is Amontillado. Poe may have known very little about Amontillado sherry, but he only needed to know a little for story purposes. The Amontillado, of course, never really existed.