I'm not certain, but I do know that rural areas have always seen themselves in opposition to the cities. As the economy in rural areas has declined, they've tended to blame city people for it, rather than changes in market forces. I think its a statement of defiance more than it is actual support for Trump.
I can only report on what I’m hearing from South Carolina but I suspect it’s not at odds with anywhere else.
-They’re furious at the perceived corruption that has rallied against Trump after his win against the odds last time. They feel that “the swamp” has made it’s blow of retaliation against Trump, and hence them. They’re flying their flags to show him their support / loyalty.
-They loathe and detest “the left”, whether that’s BLM, women who don’t know their place, LBGTQ, immigrants, socialists, reds under the bed , pro choice, intellectuals, New Yorkers, psychologists, hipsters, youngsters with their life before them and hope in their heart etc. They are reactionary to the core, and Trump scratches that itch. He says the things they feel they’ve been barred and censored for saying, from mocking the disabled to bragging about grabbing pussy to hating on the stranger in the night time, it’s Trump who is speaking aloud what they’ve been biting back for all this time.
-They truly madly deeply believe that Trump is chosen by God, and therefore a warrior chief for their own primary concerns.
- The economy apparently boomed under Trump. Ignore all the underlying truths about this, including the fact that very few at the bottom actually got to benefit from it. They were waiting for their turn, waiting for the benefits to arrive in their own neighbourhood. Having Trump’s second term snatched away means that they’ve been cheated of their access to the vast riches they envisioned cascading down towards them. The American dream of hitting pay dirt is deeply enmeshed in the psyche (whether that’s gold, oil, cattle, business, whatever). They’re enraged that the liberal elite, the book readers and the school teachers, have circled the wagons around their own riches and once again shut out the working man (cowboy, miner, steel worker, auto worker, farm hand, docker....)
They can’t act out their loyalty (they know Biden will be inaugurated) so they're demonstrating it. It’s the same thing as the South flying the confederate flag 200 years after that war was lost .
Flags are a fetish for Americans. The great majority started every school day throughout their most formative years pledging allegiance to it, like a daily prayer.
Of course the Trump flags continue to fly. This is a safe passive way to stand their ground and demonstrate common cause with others.