A bit more 1/2 voted in those elections (55% - 60%, depending how it's calculated). The highest turnouts in 2004 were in Minnesota, Wisconsin, Maine, and New Hampshire, where almost 3/4 voted. Maybe the cold weather or playing hockey gives people the intestinal fortitude needed to vote in a national election these days.
Otherwise, the tendency to vote increases the more someone resembles a campaign contributor - successful & white. As long as both major parties are in thrall to their contributors, people who have other interests and needs see little benefit in voting and the system will roll merrily along. We won't have democracy as long as a rich man's dollars are worth more to a candidate than a poor man's vote.