They got away with it because the founders, good and honest men that they were, trusted the States more than the federal government when it came to having control over elections, and never in their wildest dreams thought that citizens of the United States of America would stoop so low as to cheat in an election.
The founders didn't trust federal OR state government. The founders, in the federal U.S. Constitution, guaranteed a "Republican Form of Government". Cheating in an election does not make a republican form of government.
Please, don’t blame the founders.
BTW Framers, not founders spent the summer of 1787 crafting our once Constitution. They knew man’s corrupt nature.