Why was(or is ) the south so far behind?

Religion

That's a bigoted thing to say. You people know nothing about the South.

nullo facere opinari omnia in serium convertere. vitae ad eundem modum jocari

"White trash" suffered from parasites they picked up from the soil by running barefoot. The blacks were immune, but the poor whites were not, so they became chronically tired. That is true by the way, not made up shyt.

I forget the name of the parasite involved, but it came from Africa.... with the slaves. Talk about rough justice.

It was badly isolated before the age of the Interstates. It was much poorer, with an agrarian basis of its economy. In an agrarian society, wealth and poverty usually relate closely to ownership of land -- and land ownership in the South was extremely unequal. People could travel for miles on bad roads and see much the same thing. Few Southerners knew much else -- unless they had served in the Armed Forces.

Agrarian elites dominated politics. They resisted any industry because it might compete with their plantations for workers. They underinvested in education for all people but their own families. As with agrarian elites of other countries they were reactionary in their politics.

What forced the South to change? Elderly Northerners started traveling to Florida for the winter, and whether they came from Chicago or New York they had to travel through the segregated, backward South. They got to see the signs of segregation and got to hear a certain ugly word that begins with the letter N tossed around more frequently than a word like "food" or "money". Maybe elderly people were in no mood to protest, but they told their children and grandchildren the disgusting tales of segregation. Those children and grandchildren were often similarly disgusted. Some went to the South, especially (as relevant to this movie, to Mississippi).

Add to that -- television. The TV networks had no qualms about showing successful black people. If Southerners disliked that -- tough luck! Eventually the protests in the South against segregation and disenfranchisement drew TV cameras. ABC, CBS, and NBC could show some oily Southern pol saying that nothing is wrong and then make a cut to malnourished children. TV, unlike radio, is far more explicit in showing any violence -- and the segregationists used violence as means of repression.

Violence discredited the segregationists as nothing else could.