There's not one single reason when the election went the way it did, just like there is not one demographic that made the election go the way it did.
But the argument "It wasn't the economy. The economy is great, just look at these numbers [published by the government]!" is just politicians falling for their own rhetoric.
The US governments have been skewing official statistics for decades. I've lived through decades of "Oh, X is fine" when my lived experience has been quite the contrary.
I suspect that a significant source of D's sitting this election out is similar to a lot of R's thoughts: "You've ignored us for decades telling us that things are getting better, but they never do. We're done. Burn it down."
Maybe they don't expect things to get better after this election, but maybe their hoping that spreading the suffering will prompt more people to demand change as well.
Just a thought.