Why didn't the Allies mass-execute all SS members and Gestapo members in post-war Germany?

Only a very small amount of Nazi war criminals were executed after the Nuremberg trials. Why didn't the Allies just purge all members of the SS and other Nazi organizations? They had the power to purge and mass execute these Nazis.

Iraq is currently mass executing all captured ISIS members. If Iraq is able to do this, the Allies could have done this with the defeated Nazis.