I think the rural population plays a huge (but by far not exclusive) role in it. The Republicans aren't exactly the true defenders of the countryside lifestyle, but they're closer than the Dems, so they win (that pattern shows up for a lot of groups).
If you think the industrial crash and automation boom are bad for city dwellers, you don't know the half of it. You can just drive through some of these towns and feel the weight of despair. A new model is required, unfortunately, I don't think it will be embraced until we have no choice. Incidentally, this is also what the major incongruence is with the Deep South and all these other hyper-conservative places having strong socialist movements before later turning into what we know and fear today.
You can say racism, but people in these regions didn't exactly become not racist and then just shrugged and went back to it because Eugene Debs got arrested. Not the root cause. This kind of economic black hole fosters surpremacism, both in America and all over the world, because surpremacism is free.
If the Dems were able to embrace a radical economic shift for the benefit of...well everybody, but actually including the rural population, I think we'd see a sharper dropoff in their base than even Donald Trump could accomplish. The bread and butter of the GOP is not college-educated right-libertarian radicals who keep a copy of The Fountainhead in their glovebox and make 130k a year.