Because 'horizontal' is referenced to the local system. Linear H in Europe and linear V in Brasil generate the same signal at the satellite. RHCP will always be RHCP when it reaches orbit, no matter where it originates.
For uplink we chose this as this is kind of S-Band standard and space rated antennas are easily available. The same is true when using linear on X-Band downlink (as you might see when checking your TV satellite programme settings). We need both polarisations to decouple the WB and the NB transponder and as the local definition of horizontal/vertical is depending on your location, you need to apply LNB skew, which can be calculated here:
Thanks a lot for this excellent explanation.