Just because you have extended a courtesy once does not mean you must extend it to all. The Nazis did not produce and seek publicity of their atrocities and worst actions. The Nuremberg trials were done to ensure that those actions were well documented and widely known and a public trail was an excellent way to accomplish both. OBLs actions were already widely known and well documented. If you really want to get all legal about it, under what authority did the US try the Nazis? At what time did the Nazis commit crimes that were within the jurisdiction of a US court? The only authority behind those trials was the right of the victor over the vanquished.