An endorsement represents the ultimate acknowledgement that this campaign is over, it's a gesture of goodwill, and it's the first big step towards unity after an intra-party battle that lasted longer than anyone expected.
Sanders, some argue, dragged out Democrats' family problems by waiting so long, especially after other party notables such as Obama and Senator Elizabeth Warren endorsed Clinton weeks ago. The wait has even left some progressive leaders scratching their heads about what took so long.
With the caveat that we don't know everything that the campaigns said to each other behind closed doors, here are the most popular theories explaining the delay; it's likely that at least one of these factors may have played a role. Sanders may have dragged his feet on an endorsement because:
1 The spotlight stayed on him longer
Even after Clinton had declared victory, many of Sanders' moves these past few weeks made headlines. That's in no small part because his stubborn refusal to give up - even when the writing was on the wall - upped the endorsement intrigue.
He returned to the Senate with reporters trailing him, wondering what he'd do with his newfound stardom and asking when he'd endorse Clinton.
Add it all up, and one of the more cynical views among some Democrats is that Sanders took his time getting to the "e" word because he was loath to give up his time in this post-race spotlight.
A more charitable view is that he wanted to keep attention on the causes he fought for - and the most direct route was to stay on-stage as long as possible.
"Success in a primary can be fleeting," said Neil Sroka with the Howard Dean-aligned and Sanders-supportive Democracy for America. "There was a need to cement the important parts of his success and ensure it lives on."
2 His voters weren't quite there yet
Sanders finished this campaign with superstar status in the progressive world. His followers were so loyal to him - and at times antagonistic to Clinton - that top Democrats openly wondered whether they needed to do something big to unite the progressive and moderate wings of the party. Liberal icon Elizabeth Warren as vice-president, anyone? (Senate Minority Leader Harry Reid reportedly thought that might be a good idea.)
At any rate, Sanders may have thought pivoting to Clinton immediately after the primary would do more harm than good for the party. A Clinton endorsement so soon after railing against the establishment she represents might have rung hollow to his supporters, more than a few of whom said they were motivated to get involved in politics for the first time because of Sanders.
There's some evidence the party still has work to do on the whole unifying front: As Democrats wrapped up their party platform meeting this weekend in Orlando, MSNBC's Alex Seitz-Wald reported that Sanders supporters shouted down an amendment that would have named Clinton the party's nominee.
3 He needed time, too
In the heat of the primary, Sanders and Clinton did not have a friendly campaign. She said a lot of things about his record in the Senate that were a little loose with the facts, and vice-a-versa.
Their differences stemmed not only from personality, but from policy. From Sanders' perspective, Clinton didn't go far enough on expanding healthcare, on regulating Wall Street, on making university more affordable.
Politicians are human, too, so it's possible Sanders himself needed time to cool off and get used to the fact that he'd be voting for Clinton this November.
And on the policy front, it's likely no coincidence that the Clinton-Sanders kumbaya comes days after Clinton expanded her university affordability plan to sound a lot like Sanders' plan: free tuition at public schools for working families.
4 He wanted to cement his movement
Or at least give it the best possible chance to succeed. And success in Washington is directly proportional to how much leverage you have.
By withholding his endorsement - and the progressive cred that comes with it - Sanders theoretically had leverage to exact actual policy changes from Clinton.
He did. Democrats met last weekend in Orlando to piece together their party platform for November. The result was newsworthy: "The Democratic Party shifted further to the left in one election than perhaps since 1972," wrote the Washington Post's David Weigel, "embracing once-unthinkable stances on carbon pricing, police reform, abortion rights, the minimum wage and the war on drugs".
Sanders was not able to get Democrats on record opposing Obama's Trans-Pacific Partnership trade deal. But he did get the party to support a US$15-an-hour minimum wage, carbon pricing, more regulation on fracking (Sanders wants to ban it), a pathway to legalising marijuana and Clinton to support a public option for healthcare.
The Sanders camp got "at least 80 per cent" of what they wanted, Sanders' policy director Warren Gunnells told reporters. "I think if you read the platform right now, you will understand that the political revolution is alive and kicking."
Sanders didn't win everything (including the biggest thing). But he did win far more than anyone - including himself - ever thought he would. And five weeks after Clinton clinched the nomination, he decided it was finally time to endorse her.
