The U.S. Constitution reflects the United States of America at our founding, when there was no true model for such a document. The founders were not swayed by the organization of Britain or other nations in the 18th century or by the Articles of Confederation of the pre-Constitutional colonies. The framers were, in large part, the cream of the original citizenry, but—having lived during the revolution—appreciated the power of the “will of the people." By both formal and self-education, most were familiar with concepts of democracy from the classical world, international business, and alliance relations concepts stemming from the Renaissance and the Enlightenment. Some had represented the colonies with the contemporary governments of Britain and France.
So the times included a world in which the mechanics of finance, trade, citizenship, and treaty-making were appreciated as important. But in actual governance, they had a clean slate before them. Because we know no difference, today we celebrate that the framers evaluated Old World governance and came up with the genius checks-and-balances tripartite government.
The Constitution reflects that the times included many of the same practicalities as we see in our international world today. In its creativity, it reflects that there was no actual model to base itself on. Because of the pre-revolutionary iron fist of military/monarchial rule, rights were valued by new Americans. Because these new ordinary Americans knew how to overthrow a central authority, we have our precious Bill of Rights, which they demanded. The times generated a document of brilliant practicality.