Why are space rockets usually launched from west to east within the equatorial plane?

  

Every particle on the earth has a linear velocity directed from west to east due to rotation of earth about it polar axis. The magnitude of the velocity is v= ro. 

At equator r = R (Radius of earth) and hence Va = Ro (Maximum velocity). When the rocket is launched from west to east at the equatorial plane, this, Maximum velocity  = Ro, is added to the launching velocity. Hence, the launching process becomes easier.