Jay Cost of the Weekly Standard offers that possible explanation.

If anything, the Democratic party has only shifted toward the left, away from the Clintons, since 1996. How is it then that the Clintons are set for a major restoration, at least within their own party?

There are a lot of answers, of course, but one that I think has gone under-reported is: fear. At the least, Democrats have good reason to be afraid that their position in the electorate is not what it was just a few short years ago, and that Clinton offers an advantage in her personal reputation that no other Democrat can provide.

For a candidate who promised to undo the “mistakes” of the previous eight years, Barack Obama’s tenure looks awfully similar to his predecessor’s, at least insofar as their political trajectory goes. In his first term, Obama once had enormous political capital, lost it very quickly, and had to fight tooth-and-nail to win reelection, relying ultimately upon his core partisans. Sound familiar? In Obama’s second term, the parallels between the two have become downright eerie: external events have shattered an image of competent professionalism, exhausting their partisans and reducing their once-powerful political coalitions to a rump.

Taken together, the parallels are striking. …

… This is a problem for Democrats, compounded by the fact that they will be seeking their third consecutive term next year. Most of the time, parties lose that bid, and have always lost when the term-limited incumbent president is under 50 percent approval. It is not hard to understand why. The opposition party inevitably tags the (previously unknown) nominee — whomever that may be — as a clone of the incumbent president. That attack might not work completely, but voters who approach elections with informational constraints are inclined to see the connection.

Enter Hillary Clinton. …

… Clinton is a known quantity, which means Democrats have reason to hope that her reputation is secure enough that she cannot be dragged down by an unpopular incumbent. People know her, seem to like her, and understand (so the hope goes) that she would be a change from Barack Obama. It helps on this score that she was his rival in 2008, and we still see frequent reports that their relationship remains chilly.