No successful republican nominee has failed to win MS.
MS has been the bellweather - choosing McCain in '08, and Ford in '76.
Ford won 2, and that's the worst performance of any Republican nominee, winning only MS and TN.
Even Ford won states like ND, KS and IA.
Every Republican nominee has also always won the South Carolina primary, and that's actually slightly relevant because South Carolina always votes at the beginning of the contest, unlike Mississippi which traditionally voted very late when everyone except the nominee had already dropped out.
Ultimately, trends like these only exist until they're broken and you shouldn't put any stock in them. Remember, "as goes Maine, so goes the nation" used to actually be a thing until 1932.