The short answer is that wind turbines are placed in geographic locations where the wind is strong and sustained. This is determined by taking wind measurements and looking at historical weather data for several decades to determine wind flow in the area.
Other considerations include avoiding placement of turbines near structures and neighbors. Currently most wind turbine regulations are managed at a local or municipal level meaning that they vary even in the same state. Placement is very important since turbines do create wind disturbances or wakes for hundreds of yards downwind. For instance, if a upwind property owner wanted to install a wind turbine on their property, the wind reaching their downwind neighbor might be impacted.