It was the Founders' best shot at closing any loopholes for expansion of Federal power:
The naysayers did not trust the dam to hold. Rather, they were sincerely worried that the national government would expand beyond all recognition, and that, in its broadened form, it would inevitably begin to encroach upon the rights of the people. To help prevent this from happening, they sought a parchment backstop to which they might appeal in such cases as denied their natural liberties.
They already understood that power tends to attempt to amass more power. But how did they know that the masses would, someday, demand more? Wise men, indeed. Freedom from government was their lodestar.