I think it was beneficial for the National Socialist to keep it at a distance. That they used euphemisms like "Final Solution" has always suggested that they feared the German people's reaction had they been exposed to an undeniable truth. Having an Autschwitx or Treblinka on German soil would have been impossible to hide. With the Death Camps being on occupied territory, way away from German public opinion and German public morale, was a simple and horrifically practical means to an end.