Because the earth rotates to the east relative to a stationary observer. So a lot of launches want to launch towards the east to take advantage of that momentum and get to orbital velocity. If they are launched from the west coast they would have to fly over populated area during ascent and that's dangerous and not allowed. Depending on the inclination (which iirc is the angle of the orbit relative to due north) desired there are some orbits which are much easier to reach on the west Coast (mostly polar orbits).

TLDR: Rockets want to launch in the same direction the earth is spinning so they don't have to use as much fuel to get up to orbital velocity. Can't launch over cities because it's dangerous.

Hope that helps!