In my part of the North Carolina, roof vents–which help vent the hot air in a house’s attic–are almost universal. During a recent business trip to southern Kentucky and Tennessee, I noticed that very few homes (old or new) use roof vents. Instead, the builder opted for the turbine fans or some other type of ventilation.
Why the difference?
Well turbine fans (wind powered) and forced ventelation via attic fans will move more air then natural convection vents (most of the time), so I would wag, that they have more direct sun on the roofs (less clouds), possibly hotter climate then NC.
Usually, ventilation is an either-or situation. For instance, you normally would not see gable-end vents AND ridge vents. Passive turbines remove air well, and so do ridge vents, but I’m not sure you would want both.
Also dont forget the people designing the housing project or just one house have a certain idea and budget in mind for everything. Vents are less expensive than fans. Also custom build homes might have the option of adding a fan instead of a vent.