My info may be outdated, but I think Birmingham is an international airport and New Orleans is (Delta used to fly to Cuba from New Orleans). Even Jackson MS is set up for international flights. Ramp fees must be cheaper at Montgomery. Delta had to do some looking to find an airport that couldn’t accommodate the flights.
A different article said Birmingham had bad weather also. One of the planes was in a holding pattern near Montgomery then flew up to Atlanta, but couldn’t land and flew back to Montgomery.