hii..
there are two reasons, both are inter-related too.
1. from ground station, u have send a high frequency, with high power;and earth station can easily have power arrangements;while in satellite, one has to save as much power as much one can!!!
2. on satellite u cannot have very large diameter antennas. so, from earth station, one has to send a narrow-beam signal, according to a small size satellite antenna.. whereas on earth station it can have upto 30feet dia antennas, to collect the signal from satellite..
both points are inter-related b/c to transmit a narrow-beam signal, one has the transmitter has to send more power also..
i think u will understand the reason now..