In the South civil rights was still a current issue and black voter registration was still poor.
That. and the Southern states had a disproportionate number of military installations. McGovern was seen as someone who'd cut military spending, and the jobs and income that were generated from that. There are any number of small cities in the South whose economies are dependent on military installations, and that was even more true in 1972 than today.
The Great Plains, Mountain West, and Far West had few racial minorities, and were not motivated to vote Nixon on that basis. Additionally, the Western states had a longstanding streak of isolationism and opposition to foreign wars. The support for the Vietnam War in members of Congress of BOTH parties was much, much higher in the South and Border States than it was in the Mountain West and Great Plains states. These areas became far more Republican after Carter, due to policies regarding Federal Lands.
