In which direction you want to launch your rocket depends on the final orbit you want to achieve. For almost all orbits (except polar and a few others) it is best to launch from a spot close to the equator because the rotation of the Earth at the equator is the fastest and adds momentum to the ∆v of the rocket, thereby saving fuel. Of course, that doesn't mean you HAVE TO do that. Russians, for example, launch almost all their rockets from Kazakhstan, far from the equator, but that comes at an added cost as their rockets must therefore carry more fuel and be heavier, which is a disadvantage. Equatorial launch is best suited for geosynchronous orbits (telecom satellites) as such orbits line up with the Earth's equator at about 35000 km altitude.
The reason Sinop is ideal is because:
- Ability to launch west, northwest, north, northeast, east and southeast directions and splashdown the first stage in the Black Sea. This allows Turkey to, for instance, utilize highly elliptical Molniya-type orbits for applications like regional GPS or polar orbits for spy/recon satellites. If you launch for polar orbit from Datca the first stage would fall down somewhere near Izmir/Manisa. If you launch south from there the rocket will have to fly through Greek airspace.
- Safety -- Black Sea is less prone to attack by Turkey's traditional enemies than a spot along the Mediterranean or Aegean Sea. Datca is extremely exposed. It's very close to Greek islands Kos, Rhodos and others.
- Existing Infrastructure -- Sinop missile test range is already a military zone and has infrastructure in place (supply depots, hangars, C&C systems, radars, etc.) whereas Datca is a tourist town.
