Shocker: running against politicians who have left office is not a very effective electoral strategy.
There goes the secret Republican platform they were going to unveil in the SOTU response: Jimmy Carter screwed up the Iranian hostage crisis, and he looks funny in a cardigan, too.
On a serious note, Democrats can't quite get over the fact that this did, actually used to work. Republicans successfully ran against Democrats based on the Civil War for a few decades, and Democrats were able to enjoy similar hegemony as a result of the Great Depression.
But things didn't get quite that bad. And Obama has presided over quite a bit of the getting-bad part. That means Democrats are actually going to have to do something worth re-election.