Occam's razor says we should prefer the simpler of two explanations. I begin by reminding readers of this principle because I'm about to propose a theory that will offend both liberals and conservatives.
Theory: In US presidential elections, the more charismatic candidate wins.
People who cover politics look for political explanations for voter behavior. Voters usually vote for more simple reasons, like charisma. This is similar to how financial reporters come up with complicated morning-after analyses about the stock market when it could usually just be explained by emotional decisions.
It’s not that issues don’t matter, it’s that the party stances on issues are so engrained that it usually just comes down to charisma.
Clinton didn't represent any national shift leftward. He was just more charismatic than George Bush or (God help us) Bob Dole. In 2000 we practically got a controlled experiment to prove it: Gore had Clinton's policies, but not his charisma, and he suffered proportionally. Same story in 2004. Kerry was smarter and more articulate than Bush, but rather a stiff. And Kerry lost.
The charisma theory may also explain why Democrats tend to lose presidential elections. The core of the Democrats' ideology seems to be a belief in government. Perhaps this tends to attract people who are earnest, but dull.
It’s not that bad though. Once both parties realize it's a waste of time to nominate uncharismatic candidates, they'll tend to nominate only the most charismatic ones. And if the candidates are equally charismatic, charisma will cancel out, and elections will be decided on issues, as political commentators like to think they are now.