Hi Tom,
Of course digital rights management is an important aspect, but in fact it's the bandwidth limitation of an optical or coaxial digital connection that prevents lossless multi-channel transmission of six channels or more.
Despite their excellent sound quality, lossy codecs like Dolby Digital 5.1 and dts are very lossy. Both throw away huge amounts of digital data in order to carry six discrete channels of digital audio. I'd have to look it up but if memory serves, I believe the data reduction is on the order of 6:1. It's a tribute to just how sophisticated perceptual coding can be when it's well done.
Regards,
Alan
