2016 Postmortem

I think that was the most important factor. Dems who weren't completely on board with voting for Hillary used the hacked emails, esp the one about the primary being "rigged" for her, and either decided to stay home, voted 3rd party or didn't vote the presidential ballot line. Rs who might have been willing to vote for her, imho, went with the "both sides are awful/2 terrible candidates" meme were lost as well.

There was so much coverage of the emails, it literally drowned out any discussion of actual policy issues.

I also think Obama himself was part of the reason voters didn't turn out as strongly. He so rarely showed any fight or willingness to stand up to the Rs and others who had no other goal than to destroy him. Everything was about compromise and bi-partisanship. If he'd done nothing more than give a press conf everyday on the shameful hold up of Garland's nomination and what the seat meant for various issues like voting rights, women's rights, minority rights, campaign finance reform, employee protections, justice system reform, govt as a force for good etc, Dems would have been a lot more fired up to vote because it would've shown he was fighting for the values he said he believed.