They weren't - Clinton won a comfortable majority of primary voters. Bernie voters simply overstate how much support he had among the Democratic base.
In any case, she lost 40% of her party and really did nothing to reach back out to it. And Trump capitalized way back in April on that by planting the seed suggesting himself that Bernie should make the 3rd party run.
I share the sentiment: She lost more than he won.
What the DNC ignored was the passion and enthusiasm that Bernie and Bernie supporters had and brought to the issues and the election. Bernie filled places. Several thousand came to see him in La Crosse, WI at the Onalaska Civic Center. Only a few hundred, hand picked, supporters (were allowed and) saw Hillary in La Crosse, WI at Western Technical College.
HRC's campaign lacked passion and enthusiasm.