The US founders had to 'centralize' or create a 'central' federal government because, they knew they couldn't get the population of each state to 'go along' with the idea of 'paying back' the King of England the LOAN for the revolutionary war they just fought. So, the 'loan' from England to pay for the war was dealt with on the federal level they just created.

By the way, the 'loan' from England, was to be paid by the time the American civil war would occur. Just a coincidence?