This question was asked last year, and here was my response:
Presidential Elections - Analysis and Discussion / U.S. Presidential Election Results / Re: Why were Tennessee, Arkansas, Mississippi, and Alabama close in the 1980 election? on: September 23, 2021, 02:24:08 PM
Even though Carter's vote slipped across the board in 1980, his decline in the rural white South was not as bad. Plus, he did have an increased black vote in Deep South counties--which helped in states like MS and LA. Carter's big drops in the South were in suburban areas where they voted for him in 1976--Cobb and Gwinnett counties in GA are good examples here.
If you look at the 1980 county map he won a bunch of white Southern rural areas that would be totally out of reach for any Democrat today. Plus also Reagan won a bunch of states by narrow margins, which made his win seem like even more of a landslide than it actually was.