Because the people who have the biggest reach in telling stories about history (Hollywood) are American and the Nazis made an impact of the US via their impact

Because the people who have the biggest reach in telling stories about history (Hollywood) are American and the Nazis made an impact of the US via their impact on the Jewish people. If the US or US citizens has suffered the Rape of Nanking I imagine that we'd have more films about evil Japanese or films about Stalin or Pol Pot if your population of film makers were related to people who suffered under those despots. It may be somewhat myopic, but it's understandable. At worst, it ignores other people's pain and elevates one people's suffering as the only suffering. But it is still understandable (if lazy).