Maybe they voted against the incumbent so overwhelmingly because things are hard.
People vote based on their feelings.
When they were feeling pain, the message from the Dems was about how great the economy was, but the reality is that the stock market and GDP don't speak to the quality of life of these people. To them the Dems saying how great things were was dismissive of their real concerns.
Meanwhile, Trump latched onto their fears and concerns. Yeah, his policies are idiotic, and millions will suffer and be in worse shape. But when they said they couldn't pay the mortgage or buy groceries, he listened. The Democrats didn't because they've abandoned the working class that should be their backbone.