Many spacecraft are launched into polar orbits. For the rest, it takes less fuel if you launch to the inclination of the launch pad. The ISS is inclined midway between the main US and Russian launch sites. Antares cargo rockets launch from a mid-Atlantic launch site because it's closer to the ISS' inclination. SpaceX's new Texas launch site is as far south as you can get in the continental US, and will be mainly used for GEO satellites that want to be on the Equator.