Matthew Continetti asks that question in his Washington Free Beacon column, suggesting that perhaps it was a victim of its own success: "The fantastic wealth produced by the global marketplace enriched the center-left to such a degree that its adherents became walled off from the material, social, and cultural concerns of the working people they professed to represent. And so middle-class workers who believe a country's leadership ought to be accountable to a country's citizens went elsewhere."
We may yet find that reports of the center-left's demise are greatly exaggerated — in fact, it's flourishing here in New England in the persons of Rhode Island governor Gina Raimondo and Boston Mayor Marty Walsh. Remember, Hillary Clinton defeated socialist Bernie Sanders for the 2016 Democratic presidential nomination, notwithstanding his attacks on her Goldman Sachs speaking fees. On the other hand, she and some of her Brooklyn campaign advisers may have been examples of precisely the "walled off" phenomenon that Mr. Continetti is describing.