I am inclined to believe that less people voted because the way in which the election was framed did not inspire them to vote.
We took it for granted because it's obvious to us that 
is unfit, but we didn't do much in the way of rationally explaining why that is. Instead, we used divisive rhetoric and distilled the entire thing down to the same partisan fight that it's always been, and that made it very easy for people to disassociate and tune out.
@kel I turned 18 in 1975. I have voted in every election since then, because "civics class" taught me that as a citizen, it was my duty to be involved. The First President I ever voted for, was Jimmy Carter. I wasn't "inspired" to vote for Kerry, but it was my civic duty - so I did. I wasn't inspired to vote for Gore - but the same thing. I was more than inspired this time. A fine woman who was not a felon or rapist was running against one. Inspiration is not DUTY. And I knew the assignment.
@PirateWench @kel My first presidential election was 1988, and yes, I voted for Dukakis. I don't think I've missed a state-wide or federal election since - there were a few small ones I missed back in the early 90's, but not since. I know I've voted in a couple that had literally 3 things on the ballot.
Knew the assignment.
@PirateWench @kel PS: I live in a blue island in a red, red state.