Dolby Digital +, Dolby TrueHD, DTS-HD High Resolution, DTS-HD Master Audio, multichannel PCM, and DSD (all formats that are restricted almost exclusively to Blu-ray Disc, with a few lesser-known exceptions like DVD-Audio, SACD, and some DD+ streaming options like Vudu) cannot be carried over optical or coaxial. For digital delivery, they rely on HDMI. The problem is that HDMI also carries the video signal. Therefore, the only way to have a useful HDMI input is to also have an HDMI output (unless you assume it will only be used with players that offer dedicated HDMI audio outputs, which limits you to a handful of higher-end players like the OPPO BDP-93 or BDP-95 - units that are not routinely paired with soundbars). Once you have an HDMI input and output, you have to figure out how it will integrate with multiple sources and a display. You also need a lot of processing power to decode the lossless formats. You could support multichannel PCM only and rely on the source player(s) to decode for you, but that's going to be a source of tremendous confusion for consumers - it's not a graceful solution.

With the soundbar providing the audio processing, amplification, and speakers, you're moving into a territory where the lossy core tracks on Blu-ray (which are part of the standard) are actually a good fit. These core tracks use Dolby Digital or DTS at the highest bit rate the formats support, which has led more than a few people to suggest that the gap between the lossy core tracks and the lossless main tracks is narrower than people normally assume. And for everything that isn't Blu-ray, optical and coaxial offers the same audio options as HDMI would.