In "Night", why might have the Jews of Sighet refused to believe the stories of the horrors committed by the Nazis, even when told by one who witnessed them?
It's a common psychological reaction for people to retreat into delusion and wishful-thinking when faced with imminent danger. The truth hurts, as they say, and for the Jews of Sighet it's simply too horrible to contemplate the terrible fate that's already befallen countless other Jews in Europe. For the most part, the Jews of Sighet are fully assimilated; they've been living in the town for generations and feel at home in their surroundings. It's not surprising that so many are reluctant to leave their homes, especially on the basis of hearsay and personal testimony. There's also the practical consideration of where exactly the Jews are supposed to go. Even if they up sticks and leave, there's no guarantee that they'll find safety, not least because the Nazis' rule is widespread and growing. Many people concluded that it was better to stay put and see what happened.
Some things are too terrible to consider. If the Jews of Sighet were to believe what they are told, they then have to consider both that the same might happen to them, and that they perhaps have some obligation to take action. It is hard to want to disrupt your life; sometimes it is easier to pretend ignorance, or hope that what you are told is not true, than to face a reality that will change everything, for everyone you care about.
As the above poster stated, some things are too horrible to consider. If they believed the warnings of Moshe the Beadle, they would have been forced to take action. It was easier to dismiss his warnings as the ramblings of a madman and ignore him. This is, perhaps, one of the saddest parts of the novel. Many lives may have been spared if they listened to the warnings, but it was just too difficult to believe.