So the apathy is not with the supporters of Senator Sanders. Most voted for Secretary Clinton.
As for non-voters, either they were suppressed or did not vote at all. When the Supreme Court gutted the Voting Rights Act in 2013 and Republican states were starting to change their voting rules, the Obama Administration and the Democratic Party failed to make effective and timely interventions. Indeed, some filed cases but that's that. While waiting for these cases to prosper, what the past administration and the Democratic Party could have done is to assist prospective voters in registering them. Well, its likely that the establishment leaders believed that the money that can be used to help the facilitation of registering prospective voters on the ground is better used on advertisements.
As for those who did not vote at all, the holy grail here is that you better give reasons to participate in the political process. Give them reasons to believe that they are not simply choosing on both sides of the same coin. Show them that you're fighting for their interests. Sad to say, the Democratic Party failed their Democratic base.
If there's one thing, you can say about Republicans, they are mostly one and united in doing their thing. Look no further with the appointments of Justice Neil Gorsuch and the current deathcut bill that is being discussed in the Senate. There are maybe some dissent but when push comes to shove, they fall in line. All it takes for them is some guts to change Senate rules to allow a simple majority to affirm the appointment of a Supreme Court Justice and reconciliation to pass that deathcut bill.
Compared that when Democrats were in power 2009-2010. Democrats, could have passed a nationwide-based public option under the same rules that the Senate Republicans are doing. But they didn't, allowing the likes of Ben Nelson and Joe Lieberman to sabotage it and give them excuse to dilute it in furtherance of the interest of Big Health Insurance and Big Pharma. And from there on, people saw how the Obama administration negotiated with the Big Pharma and Big Health Insurance on writing the contents of the current Obamacare. Hail the so-called compromise. A big gift to health insurance companies where everyone are required to have health insurance or pay a penalty. A proposal to buy imported cheaper medicines from Canada was also blocked at the behest of Big Pharma. The farce is this corporately written law was not passed with 60 votes but through reconciliation when it could have passed the nationwide-based public option using the same method in the first place. Everyone saw this treachery and the rest is history.
Keep blaming voters. That will surely help retaking the White House, Congress, Senate, Governors' mansions and state legislatures.