I’m a bit skeptical about the idea of people staying home in large numbers specifically because it was supposedly safe. But I think you can get to about the same place through an enthusiasm gap.

People who might have been moved by a better candidate and/or campaign but weren’t very motivated by Clinton stayed home. It’s possible that some might have been swayed if the race was neck and neck since it would have helped drive home the stakes.

Unfortunately, a lot of people are politically disengaged, and a large portion of the population votes on vibes more than reason and policy.