It is simply this: Most, Democratic voters (not only Progressives) were absolutely terrified at the prospect of a Romney win, and therefore came out en mass to vote, if only to prevent another Bush-like catastrophe. And, I believe this was true whether or not they were particularly favorably disposed towards Obama in the first place. The alternative was simply unacceptable.
On the other hand, Republicans were not only unenthusiastic about their Republican nominee to begin with, but also did not outright hate Obama enough to come out to simply vote against him. I'm, of course, not talking about the far, far Rush Limbaugh right, or rabid racists or those Dinesh D"Souza fantasists who believe Obama is a stealth Muslim fundamentalist looking to overthrow America, etc. Yes, they exist, and make up a substantial pathological "fringe" of the Republican base, but do not (I believe) consist of enough voters to turn an entire election. These folks come out, in any event, every 4 years to vote Republican, no matter who the Democratic nominee is.
So, this is my little pet theory.
Thank you, and have a pleasant evening....