The Germans set up a network of concentration camps (and also forced labor facilities) across Europe.  Many were in Germany, but even more were located throughout the territory of Poland.  The Germans put such camps in Poland both because there were more Jews located in Eastern Europe, Poland and Western Russia and the Ukraine and also because of Nazi views of the Slavic peoples to the east of Germany as sub-human people requiring forced resettlement or elimination.  So the camps needed to be in eastern Europe.