The discourse surrounding the American Revolution and the early years of the Republic was greatly concerned with the question of how a functioning government ought to be set up without it collapsing into tyranny. Remember, one of the critical claims in the Declaration of Independence was that the British government...
The discourse surrounding the American Revolution and the early years of the Republic was greatly concerned with the question of how a functioning government ought to be set up without it collapsing into tyranny. Remember, one of the critical claims in the Declaration of Independence was that the British government itself had become a tyrannical actor, as far as the colonies were concerned. Thus, the great problem that faced the framers of the Constitution was how to create a government that could function effectively without becoming a threat to the people.
Keep in mind, the US Constitution was actually not the first attempt at creating a working structure of government for the United States. It was preceded by the Articles of Confederation, which placed political supremacy with the individual states. However, this form of government ultimately proved insufficient to tackle the problems facing the former colonies after achieving independence from Britain, and the framers of the Constitution determined it was best to scrap that system of government and instead create a new one entirely. However, if that original solution to the problem of tyranny was to place power with the individual states, thus ensuring that the Federal Government would be too weak to become an oppressive force, that solution proved to be a non-starter in practical terms. A new answer had to be found.
Thus, the Constitution had its foundations in the system of checks and balances (by which the different powers and responsibilities of government would be divided into different branches which served to counterweight one another), as well as in the system of Federalism (which divided power between state and federal levels).
In addition, however, you should recognize that the individual states (and before them, the individual colonies) actually preceded the United States as a singular entity. Indeed, during this time in US history, most people still associated primarily with their states, rather than with the country as a whole. With these factors in mind, it is difficult to imagine it being particularly realistic for the Framers to have created a centralized Nation State at all.