Source:
Clinton confidante: Sanders did 'significant damage' | TheHill
Firstly, this news article is obviously overblown --Obama went for under-the-belt tactics, just like Hillary went for the under-the-belt tactics during the 2008 primary. 2008 was far more negative than 2016. So I don't buy the implicit claim that Sanders was underhanded, or that the primary "went on too long." Secondly, I think the largest differences here are two issues:
1.) The left-wing public (meaning not just DNC loyalists, but also the Progressive Left/Progressive Independents) is exceedingly uninterested and put-off by mudslinging campaigns. In a time when people are genuinely economically hurting and have been for eight years, a personality conflict feels condescending to their needs. Bernie hit Hillary on policy and judgment issues, but they were all backed up by at least some facts and a plausible interpretation of them. Barrack and Hillary largely hit each other over nonsense, and the economic collapse hadn't happened yet, so the mood of the country was different. In the 2016 primary, Hillary tried to hit Bernie with that same kind of nonsense, too, and it succeeded for Democratic partisans --but those blows cost her among everyone else. Keep in mind that it's a general rule that if you attack a candidate with higher approval ratings, your public approval goes down. Hillary could use surrogates to viciously attack Sanders, but I don't think many people were confused over whether or not this was coming from Hillary's campaign. Everyone knew what narratives were coming from the very top (i.e. Hillary or her close advisors), and the only people who waste their breathe denying this are DNC partisan hacks or people who work for Hillary/the DNC. Like it or not, Sanders rates much more highly with non-partisan Liberals/Progressives and Independents, so the baseless attacks cost her.
2.) Hillary has this unconscionable habit of not merely going after an opposing candidate, she goes after their supporters directly. I cannot fathom this, I must imagine that I'm not privy to the right expert polling on how this helps, but Hillary's 2008 "Obama Boy" and 2016 "Bernie Bro" narratives are part of how Hillary's campaigns work. Hillary and her mouth-pieces spent 12 months going after the Progressive Left and Progressive Independents as "sexists" and sometimes far worse (a few articles, possibly funded by Correct the Record, possibly not, outright stated that Sanders supporters were white supremacists). Hillary's mouth pieces directly went after Sanders' female supporters multiple times in overtly sexist attacks, once saying that women who don't vote for Hillary are going straight to hell, another time outright stating that young women only attend Sanders rallies to find dates, another time insinuating that young women have been duped into Sanders lies because they're to young and stupid to know better, and on and on. They overtly went after all youth voters as though they were complete imbeciles who want airy-fairy, pie-in-the-sky nonsense.
So I'm not exactly certain what Hillary's team expected to happen to her poll numbers after all of this. There are consequences for how you conduct yourself in a race. I certainly went from indifference/typical anti-neoliberal dislike of Hillary, the politician, to a fervent, passionate hatred of the Clinton political machine and Hillary Clinton over the primary. I'm not certain how anyone who wasn't a Hillary-partisan would have seen the 2016 primary tactics as a means of engendering trust and approval for Hillary. You combine that with the DNC wikileaks, the pay-to-play emails, her VP choice... If you are on the Left, it's hard to have any positive sentiments aimed at Hillary. She should be very happy that she's running against Donald Trump, because it's the handicap that Hillary needs to win the presidency. So, once again, Bernie gets to be the Clinton machine's favorite punching bag and scapegoat for Hillary's mistakes and failures.