Thanks for the clarification...
There are several reasons...
1. Registration is difficult. Residency requirements vary, and while some states have adopted the Motor/Voter method of registration, it is still not uniform, moreover, not everyone owns a car to be registered.
2. Elections are held on weekdays. This is the worst time to hold elections. Most democracies hold elections on Sundays to increase turnout.
3. Voter malaise. People simply do not believe that their vote matters. In addition, there is a growing suspicion that it doesn't matter who the candidates are becuase the system itself is controlled by wealthy elites who will not look after the interests of the people (regardless of which party wins). This is what I call the "Pete Townshend Effect" -- (taken from the lyric to "We Won't Get Fooled Again" ["Meet the New Boss, same as the Old Boss"]). Augmenting this dissatisfaction is the media which continuously show the influence of powerful special interests controlling American politics. A steady diet of the rich and powerful manipulating the system reinforces the growing belief of the futility of caring about (and thus participating in) the system. THIS IS A HUGE POTENTIAL DANGER FOR DEMOCRACY IN AMERICA.
4. Frequency of elections. Americans have more elections than almost any other nation on earth (Federal, State, Local, Special). It contributes to voter fatigue.
5. Systemic Overload. Increasingly, people are overloaded by the idea of governmental complexity. The idea of trying to understand government is like scaling Mt. Everest, and many are simply giving up the attempt even to try.
6. Basic Nature. The Americans are one of the least political people on the planet. Other nations have very politicized populations. But in the USA, people have always tended more toward personal, social and familial remedies. Thus, people tend not to think of the government first. There actually is a much more deeply ingrained sense of self-reliance among Americans than among other industrialized nations (though this is changing with time, it is still true).
7. Basic Structure. America was not conceioved as a democracy, but a republic. People technically don't vote for presidents (the electoral College does), and people never vote for Federal Judges. That's two thirds of the Federal government. This fact tends to dampen enthusiam for voting.
8. Basic stupidity. I'm not trying to be insulting; there is much evidence to support the contention that the typical American doesn't conceptualize candidates and issues very well. The parties know this, which is why they spend so much time trying to "market" candidates like toothpaste. They ignore real discussions of substance in favor of soundbites and photo-ops. The American people have simply developed a good "sales resistence."
Hope these ideas help.