“We still do not have a comprehensive understanding of why this happened”
It’s far simpler than most would like to believe: prior to the Clintons occupying the Oval Office during the ’90s, the Democratic Party routinely complained that it was outspent by Republicans. This was almost always laid at the feet of “corporate interests” (often accurately, sometimes not), in advancement of the classic “we’re for the little guy” pitchline.
Bill Clinton’s scandals first caused the party to close ranks, and then when Lewinsky broke they completely circled the wagons. The Clintons’ character — yes, for both of them — became literally unimpeachable in Democratic circles. To do otherwise was to allow chinks to develop in the party’s armor. His removal from office was impossible as long as the party held its line, with impeachment itself being laughed off as “just another Republican smear campaign”. The actual crime Clinton was found guilty of in a court of law — lying to a judge in order to cover up his sexual history during a sexual-harassment case, for which he was fined and disbarred from legal practice for several years — has likewise been buried and forgotten in left-leaning circles.
But the REAL legacy of Lewinsky was that the Clintons were able to sob-story the party elites into setting up a legal defense fund. For the greater good, of course. The Clinton Foundation became a financial powerhouse over the next few years. This was also the beginning of Clinton-loyal activism sites like MoveOn.org. Thus was built a strong network of political loyalists and financing, which also worked closely with the DNC to advance other Democrat candidates.
All loyal to the Clintons, who were already notorious for keeping “loyalty lists” regarding those who pleased or offended them.
In short, the Democratic Party invested so much of its political capital into defending the Clintons that ANY critique with bite to it is responded to on a visceral level. Until Hillary’s failure in 2016, they remained the King and Queen on the party’s chessboard. Which is why there appears to be no one of substance remaining to replace them… no one in the party ever dared THINK in those terms until now.