Because they're launching on an Ariane rocket, which is a French rocket that launches from French Guiana.

As to why they chose that, I have no idea

> The rocket had a streak of 82 consecutive successful launches between 9 April 2003 and 12 December 2017.

That's why.

(We also like international cooperation. We build the telescope, ESA chips in with the launcher. Same situation with Orion and the ESA's service module.)

You get an advantage launching closer to the equator. The velocity of the Earth at the equator is higher so less deltaV is required from the rocket itself when compared to launching somewhere closer to the poles.

You get a speed boost the closer you are to the equator and if you want an equatorial orbit you have to spend fuel in orbit to correct down to zero degrees.

It is way cheaper to correct these things on the planet than with bigger rockets.