The Meaning and Reality of Individual Sovereignty

Why has the idea of individual sovereignty been granted such primacy of place in free Western societies? Why were the men who formulated the Declaration of Independence willing to declare: “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness," and, further, to insist "That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed." Why, as well, are ideas of individual sovereignty, which have proved so successful in guaranteeing freedom and prosperity in the West and, increasingly, around the world, under attack, by those on the left and the right who offer collective identity as an alternative?