Most people don't know what the new investigation was even about. All they will know is that the "partisan witch-hunt" led by the FBI will have failed its plan to unfairly prosecute one of the greatest, most honorable women of our time.
I can't imagine anyone having such a rosy view of Hillary beyond the core group of (I can't believe I'm about to say this - the company I keep must be rubbing off on me) diehard feminists who have been with her for every step of the way. Most people
disapprove of how Hillary has handled the entire email situation, along with bafflingly viewing Trump as more honest than her, despite the fact that he's objectively lied far, far more than she has throughout the race.
That first exoneration didn't really stick, so they wanted to have another one right before the election to make the sheep feel safer about selecting their next executioner in chief
If that were the case, then why not just announce that after further review, they're sticking to their decision to not charge her? Why first throw the race into chaos with the vague letter declaring the investigation back open? That was hardly going to reassure anyone.