This is really interesting. I think I know the answer. I live in Louisiana and I noticed a majority of residents grew up here, had family that grew up here, and have kids that are all growing up here and everyone knows each other, all their kids know each other,and there's very strong community values and regional pride.
So my theory is that happiness is tied to roots and community more than good economy, good education, good employment prospects, good future for your kids, low crime, and all of the other things LA isn't really strong on.
I would like to see the happiness research compared to migration statistics.