Air condition and productivity.

From A tale of two American cities by Ed West.  The subheading is How Austin is winning the battle for talent. 

For all that, few inventors have had such an obvious impact as Willis Carrier, who in 1902 came up with the first air conditioning unit. At the time of his breakthrough, GDP per capita in the southern states was 40 per cent lower than in the north: the old free states had long been dominant economically and accounted for nine-tenths of American industry at the time that South Carolina seceded from the Union, dooming that romantic but wrong cause from the start.

Air conditioning changed all that, and as a result American life has become far more orientated towards the South.