To prevent from succeeding.
Montresor tells all of his servants that they must stay in the house. He also tells him that he is going out and will not return until much later. Since the story takes place during a popular festival, the servants want to leave and join the partying. Montresor knows that if he tells the servants he won't be back for a while the servants will leave the house anyways. Since he is not there to make sure that they are doing what he's asked, the servants will go off and do whatever they want.
That's cute but eggnog is nasty, I said what I said
