Kourou is in french Guyana, a french (or European) territory.
The center was founded in 1965.
It was a base very close to the equator, with an ocean east and north of it! Besides, no hurricanes....Ideal for commercial launchers.

Europeans had to decide what to do about space in 1972. The Germans and the Italians decided to cooperate with NASA on Spacelab; the french decided to build an Ariane family of rockets.

Arianespace benefitted from NASA's mistakes (too much Shuttle dependancy) . Ariane is something Europeans are proud of!

Ariane 5 has a specific market; there was an offer to launch Soyuz from there too and because it's a different market, it was agreed the Russians would work together with Europeans. Vega is an even smaller launcher. Here are pictures from a friend of mine who's at Kourou.