why did holocaust occur and why did so many turn their back or close their eyes to the reality of the holocaust?

Historic antisemitism, the rise of eugenics and nationalism, the aftermath of the First World War, the rise of the Nazis, the role of Adolf Hitler, the internal operation of the Nazi state, the Second World War and collaboration all played key roles in the timing and scale of the final catastrophe.

Hostility toward or hatred of Jews as a religious or ethnic group, often accompanied by social, economic, or political discrimination. (antisemitism) is most likely the reason people had turned their backs on the holocaust.