Some could chalk it up to schizophrenic voters having short memories.
However a case could be made that its their memories that lead them to vote the way they do.
You see, the common myth was that Republicans were "booted out" because of the economy. But that simply is not the truth. They also state that it was due to wholesale rejection of conservatism, but that wasn't exactly the truth either. The republicans began their loss in 2006 NOT because of issues with the economy but rather on the shoulders of the anti-war movement. The anger was towards Republicans and Bush's actions and policies, but it was not an abandonment of conservatism as the mainstay American voter mentality with a switch over to liberalism that caused it. Democrats won in 2006 by running moderate, and sometimes even somewhat conservative leaning, blue dog democrats that captured seats a true liberal democrat would've had a hard time getting.
However, once in power what did they do? Have the Wars stopped? Iraq's continued in line with the Bush time table, Afghanistan is ramped up, Gitmo is still open, the War on Terror still happening. All the while rather than identifying the moderate wave that brought them into power their focus was on universal health care, stimulus, bailouts, cap and trade, potential expiration of the Bush tax cuts, and other generally left focused ideas.
No, a legitimate argument could be made that its not the LACK of memory that has caused this, but having a memory. They remember they voted for Democrats to do a job with the war, and it wasn't done. They remember they swallowed thier nose and voted for "moderates" and all they saw was left, left, left. And on top of that, the economy that they weren't thrilled with in 2006 and especially in 2008 hasn't exactly got better in their eyes, but perhaps worst.
So if you swallow your nose and go for "moderates", if you willingly ignore other issues because the war's so important, and NOTHING is done to make those choices seem worth while....well, that memory is going to remind you not to try that again.
You can make a case that voters memory is poor...but you can make a pretty compelling one that they have the memory of an elephant.