There are reasons for Election DAY. Here are just a few that come quickly to mind . . .
First and foremost, the campaigns don't end until election day. Every election year we find out more and more about candidates for public office in the days before the vote takes place.
Second, the chance of fraud rises with early voting. Common sense and logistics tells us that much. Without Santa-like powers, it would be much more difficult to travel from place to place, state to state, poll to poll, casting multiple ballots in a single day, than over whatever time period early voting was allowed. Is it any wonder that St. Louis City Democrats (Mayor Francis Slay) advocate early voting?
Third, early voting allows the votes of folks who, may they rest in peace, are no longer citizens of the United States but of the Great Beyond to be counted in elections. For example, link here. Maybe I'm cold and unfeeling but shouldn't elections be decided by the living?