I think a lot of the population increase of the southern states can be traced back to several different factors, not just necessarily AC. For example, Union labor is much less common in the South, so companies have a much easier time getting things done. There are lots of other things that I think are "pro-business" but also we're simply less regulated as an individual and also much of the time less taxed.
There are just natural things that exist in much of the south that you simply cannot reproduce like oil, which leads to oil refineries, which needs shipping which needs ports. Last I checked there is no ocean front property in Ohio and many of the more northern states.