Why do registered voters end up not voting?

[Progressive Pessimist]

1. They die. In that case they had better quit voting!
2. They move. Their registration in Pennsylvania may have not yet been cancelled while they vote in Virginia.
3. They become too senile to make electoral decisions.
4. They get indecisive on whom to vote in one election or another and decide not to vote.
5. Dislike of both candidates.

6. They just don't end up feeling like it (rain, being sick, etc.).
7. They forget.