Christmas is barely the last hold out for holidays where the people who need time off to vote (retail, food service, other low wage high hour jobs) get at-least a bit of time off and that is being eaten into each year.
Do you really think a federal holiday for voting would do anything but start a whole new round of election day door busters that make it even harder for these people to vote?
A voter holiday is a waste of political capital that doesn't help the people who would need it. Pushing for the voting booths to be open all month long and to allow no justification mail in ballots are the kind of valuable reform we can push for rather than a symbolic holiday that would be ultimately worthless.