During World War II, the Nazis were in control of much information and spread false information about what was happening to Jews and other minority groups. They also punished people who tried to spread the truth. The Jews in Elie Wiesel's town lived far from the war and the first concentration camps and didn't know what was happening. When the Nazis came to their town, it was too late for them to escape or find out the truth. This was partly because they didn't have access to information and partly because they couldn't believe the terrible things that were happening.

Thank you!

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Information control/suppression. Some people had no idea about the scope or nature of the death and concentration camps until they were literally there; there was a deliberate propaganda effort to not raise alarm about it by pretending it was an inoffensive "relocation." "One booklet printed in 1941 glowingly reported that, in occupied Poland, German authorities had put Jews to work, built clean hospitals, set up soup kitchens for Jews, and provided them with newspapers and vocational training." (Holocaust Encyclopedia)

Later in the war, it was much harder to hide reality. But as you know, some ignorant people deny it even today.

Also worth to note that most of killing in concentration camps started in 1941 (https://en.wikipedia.org/wiki/Final_Solution) and even then it was hard to prove it because were no cameras everywhere like today, so it was word against word.
And most people tend to not believe scenarios that they are afraid of.

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In the specific case of Wiesel, the simplest answer is that the Hungarian Jews weren't being murdered en masse (at least while they were still in Hungary) until very late in the war. It is actually true in general that the mass murder of European Jews by the Nazis didn't really begin until the middle of 1941.

As you point out, Moishe is deported to Poland in 1941 and barely escapes the mass murder being perpetrated against the Jews there. But when he returns to Hungary to warn the rest of the Jewish community there, he is ignored or not taken as credible. The specific reason for this is going to be different from person to person who didn't believe him, but it basically just comes down to denial. That is, although the Germans had spent years demonizing Jewish people, before roughly mid 1941, they actually weren't murdering them on a large scale. Of course the Jews were rounded up into ghettos and concentration camps, but those weren't actually designed or operated with the intent to kill everyone there. It was only after the invasion of the Soviet Union, and later the Wannsee conference, that the Nazis decided to kill as many Jews as they could, not only in the territory they seized from Poland and the Soviet Union, but also in Western and Central Europe.

In that historical context, Moishe's warning is a very early sign of what is about to happen. Unfortunately, in many disasters, the very earliest warnings aren't taken seriously. It is almost axiomatic that any power waging war on another will have at least isolated incidents of war crimes. It is therefore possible, in 1941, to write off even accounts as horrible as Moishe's as isolated incidents which were abominable but which wouldn't have indicated that the German policy was now to murder all Jews.

And remember, until late in the war (1944), Hungary, which is where Wiesel lived, wasn't occupied by the Nazis, and the Hungarian government didn't deport Jews to concentration or extermination camps in Germany and elsewhere. Because Hungary was a German ally, it might have been tempting, even after hearing about the monstrosities occurring Poland and Russia and even in the rest of Europe, to believe that as a Jew in Hungary, you would be safe from Nazi persecution. After all, why would the Nazis invade their own ally?