It was close but it was the voters who chose not to vote that determined the election, in 04 Bush had 63 million votes to Kerry's 59 million votes. This election we had 6 million less people vote [57 million for Romney and 59 million for Obama], its safe to say that Obama's voters turned out whereas alot of Republicans were either turned off by Romney and chose not to vote at all or chose to vote for the "other parties" [the other four we never ever hear of].
IMO the lower turnout is actually the voting numbers returning to where they were before the computer aided politicking.
In the 1990's some large mail order companies ceo's approached the Republican party and told them they had very interesting information on people that before the computer was unknown. In fact, they could largely identify what party and what issues many people were sided with. For the first time individuals were targeted. And they were targeted in a way to motivate them to vote. So in the 1990's people started getting individual mailings saying so and so wanted to take away your guns, or so and so wants make abortion legal, or so and so will raise your taxes. This motivated people on a single issue to come out and vote.
It was what gave the Republicans the House in the 1990's. By 2000 it was used to elect Bush.
What was really happening was the Republicans had substantially grown their party by bringing in "new" voters who were stay at home Republicans.
By 2004 this tactic reached its peak when Republicans went to the polls believing Saddam had attacked us on 9-11
What has happened since 2008 is that on a Presidential level the Dems have finally started to catch up with the Republicans. However, the Republicans still lead them on a local, House of Representitives level where a single issue may be all anyone knows of thier rep.
So, now the Dems turnout and the Republican turnout is returning to the normal, or pre1990's levels insofar as the relationship between them.