Why Do We Vote On Tuesday? --short answer is that farmers liked it that way. Lots of bellyaching in the comments about how voting procedures need to be modified to allow more inclusiveness.
Shying away from What We Expect Mojo To Say (i.e., "Making it easier to vote inevitably will lead to people taking voting for granted," or even, "Why do we want everyone to vote? "), let me take a different tack --one that WordPress inexplicably refused to let me post (?):
[In response to the idea that Election Day should be a national holiday] NO! to a holiday for voting. I submit that making it a holiday will actually depress voter turnout rather than enhance it. Consider: the pressure would be enormous to make the holiday on either a Monday or a Friday to avoid (further) disrupting the work week. How many people take Memorial Day Monday to go pay their respect to the war dead, hm? How many people go on Labor Day to see the unions parade? Voting Day would turn into another wasted workday, with added absenteeism before and afterwards (honk if you've ever taken off Friday before a Monday holiday weekend, or Thursday before a Friday holiday weekend). And since voting wouldn't take all day, retail stores would be encouraged to run sales to attract the off-work consumers (screwing the employees who have to work that day, natch). The easier and cheapest solution across the board is to extend voting hours in all jurisdictions to 24 hours beginning 5AM EST on Voting Tuesday.