frequency is simply the number of cycles a signal has completed per unit time.
unit of measurement is CYCLES per SECOND or Hz...
for downlink, the frequency is lower because lower frequencies could reach a greater distance given the same output power from the antenna. lower frequency would result to a lower Free Space Loss, absorption loss, etc...
And way above there, in the satellite, we actually need to conserve as much power as we can to keep the system running for a long period.
On earth stations, we can afford to transmit more power using the higher frequency.