The closer to the equator that you launch the more of an advantage you get from the rotation of the earth. Your starting velocity is higher launching from the equator than from a point closer to the poles of the earth.

It is launching on an Ariane 5 rocket which has the launch facilities in French Guiana (for the reason I mentioned above).

Edit: Vandenburg is also even worse from this perspective because in order to launch out over the ocean, you are launching against the rotation of the Earth so you have to overcome that velocity. But Vandenburg is often used for polar orbits where the rocket launches almost due south from Vandenburg over the ocean.