Slowing down / preventing the development of Air Conditioning would certainly help.
If you want to stop population growth in FL, AZ, TX and CA, you need to curtail multiple developments.
* Air conditioning, Freon (1930's)
* Malathion an other insecticides for insect control (1940's)
* Kroll process for titanium (1946), essential for jet aircraft
* Cheap gasoline
* Suburban sprawl
Confined to railroad travel, major league baseball does not make it west of Kansas City and Minneapolis. Dallas/Ft.Worth
might be able to support a team.