They mention a few major influences, and population density is one of them. In areas with more sprawl and land, it's more likely for people to drive longer distances. (This probably explains the Midwest and West)

They also mention poverty being a factor, where it's more common for families to live together, or very close, in order to help support each other. (so probably explains the South)

Another thing to consider is grandmothers helping when couples have young children. I bet if we overlayed a map of locations where people are more likely to have kids, we'd see a trend too.