This sounds like the most likely cause. People who would've voted Republican knew they were backing a loser, so why bother.
There were a ton of first-time voters, though.
I posted this already but was ignored>
Comparing this election to 2004 is sort of a false comparison because the 2004 election rate was so elevated. 20 million more voters voted in 2004 than in 2000. That election was a true nail biter and people felt there was more at stake.
2008 numbers were pretty close to 2004.