Why is it convenient that the attendants have absconded to celebrate carnival?
It could have been very inconvenient for Montresor if any of his servants had been there when he brought Fortunato to his palazzo. One of them might have seen them together, and he didn't want any witnesses. Evidently the two men had to enter through the front door and go through several rooms before coming to the stairway that led down to the wine cellar. Montresor feels sure that none of them would be there to see him and Fortunato together on that fateful night.
There were no attendants at home; they had absconded to make merry in honour of the time. I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house.These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned.
Not all household servants would be so disobedient. Montresor is a poor man. He has to have some servants in such a big house, but they are probably disrespectful and lackadaisical because he can't afford to hire the best class of servants and probably can't always afford to pay them their small salaries. They do pretty much as they please because they know he can't fire them.
Several things have to break just right in order for Montresor to pull off his perfect crime. Fortunato is drunk. Montresor makes sure that Fortunato doesn't have an "engagement," that he isn't expected anywhere that night. Fortunato has a bad cold, which will keep him from asking too many questions while they are on their way through Montresor's catacombs. And all of Montresor's servants have "absconded to make merry."