A number of states specifically structured their tax policies to be super friendly to retirees, so some ran the numbers and said “oh I can afford to sell my expensive house up north, move south and buy a better/similar house for less and pay less taxes as I withdraw my retirement money? Also I don’t have to deal with winters anymore?! Sign me up!” And since they don’t have to worry about job availability they can live basically anywhere that has the amenities they want and fits their budget, thus the American south became a gigantic retirement community