One big and important reason was that, The Great Cyrus, didn't fight to expand his country, most of the times he did it with kindness and with justice. Plus he gave a part of the city etc to them too at let them to rule it themselves! If he fought with others, he tried to fight with the least death and bleeding. Another reason was that, most of their kings, specially Cyrus and Darius were with layout. An important thing is that even now, in the 21st century, an inscription from the great Cyrus, is the the United Nations, that the meaning of it is: All of the people in the world, from any nation or religion etc., are the different part of one person, in fact everyone should be kind with each other and don't fight. When they conquered Babylon, the great Cyrus free all the poor people that were in person for a long time.