It's because 5.1 through coax / optical is encoded in Dolby digital or DTS which are standard formats that everyone has agreed on and is well documented.... Theoretically sacd and dvd-a could be sent through one cable, but there's not been one standard created for transferring the data... so the simple workaround is to let the players output the high res format to analog and then connect the individual channels to amplifiers (or 5.1 input on a preamp or reciever)....