It seems that the orcs are physical representations of evil and chaos. Sauron himself seems to resemble satin or god of evil on (middle)earth with the power "see" and bring all that is evil together. Even the “sinful” men try to come to his aid for example Boromir. The orcs of Mordor are his minions/ death bringers; they will not turn on their master because he is the only reason for their existence; the only one that can give them propose. Without Sauron the orcs would not have common goal and thus would turn on themselves.
One the other hand you have Saruman and the uruk-hai who seem to represent the evils on an industrious world and the faceless army of a dictator. To me Saruman himself could represent Hitler or any of the WWII dictators of Tolkien's time. He is someone with a sliver tough and who is able to gain influence and bend the will others with the power just of his speech. Moreover he cares not for the world he lives but seeks to destroy it in oder to bring it into the future to and create his own ideal utopia just as Hitler and his allies attempted to do in second world war. Like Hitler he obsessed with perfection.