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...

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.