A really big problem with satellite technology are the harsh environments they go through. You're dealing with something like 10-20g's of vibration peak-to-peak on ascent, and then you subject it to the freezing colds and searing heat of space, all while doing this in vacuum.

Since those clocks need to be super precise, it's easier to have an accurate reference on the ground where everything's not getting shaken and baked to bits.