HDMI absence

Maybe because HDMI is mainly a TV thing?

HDMI is commonly used for both multi-channel audio and video/audio in a blu-ray player and Cable set-top TV boxes.

For two channel audio, i2s protocol was adopted. But to this day, there is no standardization with i2s protocol. Some manufacturers are using HDMI and some are using Ethernet ports to convey digital audio between two compatible devices. Unless you buy two compatible devices that uses i2s protocol from same manufacturer, you are very likely to run into ‘hand shake’ issues.

For example, Rockna is using i2s protocol via HDMI ports to convey digital audio between their streamer and DAC. Whereas AQUA is using Ethernet port between their CD player and DAC.

The Topping D90 can interface with various IIS devices but not HDMI standard. 

Because it costs to license it and they doubt that it will appeal to too few buyers to justify the expense.

As mentioned above, the “hand shake” is an issue, and as any real transfer quality it just sucks. I hate HDMI, unreliable connections, poor overall communication etc. nice concept, crap execution. But, it is here to stay so it would seem. If you use it, run good cables, it makes a difference, and check your compatibility with both the cords and the devices. If your cables are cheapies and your run an Apple TV you might get security issues as well. (Won’t run shows from HBO etc)...

geof3, your statements are mainly for video usage. For 2ch Audio, I2S (with a compatible handshake of course), like ethernet, is generally a superior flow of signal compared to coax, XLR or optical.  

jriggy, you bring up an interesting point, namely, is the data flow superior among the various means it can travel from A to B -- HDMI, I2S, coaxial, AES/EBU, or TOSLink?

"generally a superior flow of signal"
What does that mean?
How is it measured?