I think it's because Bernie is ultimately more of a stand-in for a long simmering internal feud within the Democratic Party that the DLC won thirty years ago, and Clinton and Obama's victories papered over, that is now coming back as part of the self-evaluation period of a defeat, especially a shocking and surprising defeat like this and especially a shocking primary like this was. Bernie's ballot support essentially didn't exist outside Iowa and NH until two months before 2016 and he arguably got stronger as the primary went on (especially in terms of fundraising) even as his chances evaporated.
Bradley, Romney and Gingrich never represented a stark ideological division within their party (and Santorum was arguably the runner-up to Romney in 2012 and also didn't) especially with Romney coming back four years later to get the nomination. Dean didn't really either other than it was a preview of how Obama would be able to use his not being in the Senate during Iraq against the rest of the field which had to make those votes. Plus he became the head of the DNC and they won back Congress.
Reagan/Ford forty years earlier led to this debate in the Republican Party that lasted pretty much until Reagan beat Carter. Ford was more of a conservative than those who ran the GOP for the prior thirty years but Reagan flanked him and made him a stand-in for the moderate-to-"liberal" Republicans like Rockefeller, Mathias, Baker, Richardson, Weicker, Nixon, etc. much like Bernie made Hillary into a defender of the DLC/status-quo versus a more progressive "vision" in a way that prior runner-ups hadn't because none of them were ever as far "left" within the Congressional spectrum as Bernie is. Dean, Edwards, Bradley were all DLC/moderate types who took one or two issues to the "left" of the field (and arguably Edwards only did this in his disastrous 2008 incarnation) but in other cases were often to "the right" of the field.
In the case of Reagan/Bernie, as they were in unique positions politically, they could essentially just swing entirely in that direction. And with Ford trying to defend his record, and Hillary effectively trying to defend the Obama record plus her husbands, they locked themselves into positions that let them get hammered by an ideological wing within the party even more so than even the wing might truly support because they expected to hold territory for the general. Hillary even more than Ford was trying to out maneuver everyone to a safe center that she could drift "left" from, which would have been great, except she went up against Bernie and Trump who had no problem with assaulting these positions for being centrist or moderate or what have you rather than getting say Rubio or Cruz who would try to paint her as socialist and to the "left."
Bernie's the stand-in because who else is going to be that? Nina Turner?
Elizabeth Warren and Sherrod Brown are up for re-election so they can't exactly run out there as carrying the progressive banner against the Democratic Party itself. Even if they're likely to win. And anyone who wants to go for 2020 isn't going to do it except in the half-assed way Martin O'Malley tried to.
Which leaves Roque "Rocky" De La Fuente. And Geoffrey Fieger.
