The answer is not what you think. It's an industry decision. If you send your content to your HT 5.1 system FIRST then it can render the 5.1 and send the HDMI video onwards to the HDTV.

However it was decided that devices may not forward 5.1 Audio. This has been been discussed at length and the only time I see a HDTV output 5.1 is when the content is OTA and sometimes USB.

There is no technical reason this can't be done but the industry, HDMI and HDCP standards are at play here.

Hope this helps,
Bob