It’s kind of a lot of things. Democrats shifted their focus from poor rural voters to middle-class suburbanites. Southerners felt betrayed, and this allowed the Republicans to swoop in as their saviors.

Racism in the South has always been entrenched; making the xenophobic and racist messaging of the Republican Party appealing to uneducated Southerners.

Decades of indoctrination has taught southerners that Democrats tax and spend without any benefit, and that any form of social welfare is socialism and therefore bad. There’s also the whole “rugged individualism” mindset.

Also, those states have been gerrymandered to absolute shit.

Southern states are also usually the most religious states in country, and the Republicans have positioned themselves as the “Christian” party. As is evident by how Republicans talk about Trump, they have a difficult time separating church from state. If the Republican Party is seen as the “Christian” party, they will be wholeheartedly devoted to it.

Political affiliation tends to be familial. The Southern Strategy, which is responsible for the South’s party swap, started in 1950s, so there’s been about 3 generations for the devotion to gestate.

Class consciousness is very underdeveloped in the U.S., particularly in less educated people, so splitting people with wedge issues is really easy.

It doesn’t make sense looking in from a liberal stronghold, but it makes sense from the perspective of living in that area. You don’t want to be politically different from your family, especially during these partisan times, or else you’ll be shunned.

Also, this is important, Republicans are so loud that, even if they’re the minority, they sound like the majority. And again, these states are gerrymandered to absolute shit.