The reason is because in 1st person point of view we can see the psychotic nature in which Montresor describes all he did to Fortunato. He explains in crude detail how he killed him with no remorse, he even seems to delight in the acts which he did. For example: when he describes the walls filled with a type of moss, he's already putting inside Fortunato's head that he will have to live and cope with these toxic fumes that will poison his lungs little by little, Fortunato doesn't know it yet, but, he is going to die. And, putting this images in the head is preparing Fortunato for the worse to come, he's preparing Fortunato so he loses all hope. Montresor describes everything like a sociopath would, if it were in third person point of view we wouldn't know what Montresor was personally thinking or feeling only partially.