http://en.wikipedia.org/wiki/Election_Day_%28United_States%2...
http://usgovinfo.about.com/od/thepoliticalsystem/a/whenwevot...
I point this out to anyone who complains about how archaic some things in the UK or Europe appear to be.
However, in this country there is a constant struggle between trying to let more people vote versus trying to restrict voting. Hence why a lot of people are up in arms about potential voter fraud. The traditional wisdom is that more voters helps one party, fewer helps another. With this as a factor, you can see how it would be very difficult to change the day of voting to make it more convenient for people.
The date is set by an act of Congress from 1845.
http://memory.loc.gov/ll/llsl/005/0700/07590721.tif
I think my other arguments still stand though...