Three possible, and not contradictory, reasons.

#1: The Nazis had found, through such things as Reichskristallnacht and the T4 program, that the German people would occasionally speak out against some of the more PUBLIC violent acts of the Nazis. Hence, death camps being hidden was a way of making what was probably known to many more palatable.

#2: Chelmno was created to exterminated the Jews in Reichsgau Wartheland (Polish lands annexed by Germany). This camp was inside of Germany if lands annexed are included but is inside the pre-1939 lands of Poland.

Belzec, Sobibor, and Treblinka were all camps used in Operation Reinhard. The purpose of Operation Reinhard was the extermination of the Polish Jews. Therefore, it made sense that these camps were in Poland. Majdanek work camp had gas chambers built to kill those no longer capable of work; however, at a number of dates it seems that large groups of Polish Jews were sent to Majdanek and immediately killed.

Finally, Auschwitz was also inside the annexed lands and was therefore inside Germany. It began as a work/concentration camp. However, its location on rail lines and its eventual size made it useful for the extermination of Europe's Jews.

#3: The camps were under the jurisdiction of the SS. In the structure of the Third Reich, the various leaders tended to compete for control over different tasks or areas. Himmler had gained control over security mechanisms including the camps. However, power was more easily established in the conquered territories than in Germany where other leaders had already gained control over specific activities. Hence, the camps also worked as a method of establishing SS control in conquered territories.

EDIT: I don't know what the heck happened to the font. Sorry!