Why does USB feature so much in discussions about DACs when the newer HDMI seems better?

Many historically early electronics included USB because it was cheap. Unfortunately many audiophiles got hooked and believe mistakingly that it is a good  way to connect a DAC. It is not. I2S came along in quality devices and before that HDMI. Both are superior for audio. But for those who disagree… fine. You use the connection you like or have. Many people use computers for their source and connection through USB to their audio system. I don’t knock anyone’s setup. If it sounds fine…. there isn’t something to fix.

I am not disagreeing that HDMI might be superior in some ways, but I think the answer is that USB works. And from an engineering and cost standpoint it would be difficult to make the case for using HDMI. More wires does not mean better, USB is more than capable of providing the bandwidth for hi-res audio. 
 

As others have stated, HDMI connections have been popping up on DAC’s to accommodate I2S, not because it is inherently supported by the HDMI standard, but because I2S requires more wires to pass the signal. 
 

I will step out on a technical limb here, so please feel free to correct me, but USB audio uses a packet protocol similar to most digital communications systems. Whereas I2S is a low level hardware signaling protocol meant to be used on short buses between hardware components. Is it better? Very possible, but it is totally up to the implementation, which for now has no standard, and may be prone to corruption if not done well. USB, by virtue of using a packet protocol has some verification capabilities to ensure accurate data transmission, I don’t know, but I am guessing the I2S signaling does not have this capability. 
 

Back to the answer, they use USB because it works and makes technical and economic sense. Again, not disagreeing that I2S might sound better under the right circumstances, but if you ran the same packetized protocol on HDMI that you run on USB, there would be no inherent advantage, and it would be overkill. 

I am surprised no one has mentioned the 5v power typically carried in USB and possible noise introduced as such.  My DAC is optimized for USB, as I use a separated high quality clock connected to the DAC.  Also, I turn off (defeat) the power in the USB output in my Auralic Aries Streamer (a nice feature).  In my old DAC (Tambaqui), AES was the way to go to optimize the output of my Grimm Streamer.  Now, USB without power is  the (recommended) way to optimize my new DAC / Clock.  And clearly in my system, and to my ears, quality cables of either type make a big difference.

Could HDMI have been implemented to achieve both of these results (by the manufacturers)?  Don't know.  But I have used what I think is best for my specific situations. 

Digital discussions are funny.  Perhaps 1's and 0's lead folks to think digitally.  But our hobby is not a yes / no science.  Analogue gear is always discussed in gradients and subtleties, yet digital is often discussed as right or wrong.

Yes I know I did not discuss strengths and weaknesses of HDMI vs USB, only to attempt to discuss how USB can be the best for specific situations.

Happy Easter!

 Analogue gear is always discussed in gradients and subtleties, yet digital is often discussed as right or wrong.

Maybe there's a reason for that?  Albeit I would perhaps frame it as analog logic doesn't apply to digital. 

On the original question, I would definitely love for there to be more HDMI (not referring to I2S over HDMI) compatible DACs; especially more cost effective multi-channel DACs, and if that fed into more HDMI outputs on streamers, all the better.  

But I don't have an issue with USB DACs...these days there are just lots of good cost-effective USB DACs. 

IMHO the answer to this question lies not in the connection technology so much as the market acceptance of multi-channel music.

Stand-alone DACs probably are only in 1% of all stereo systems, and multi-channel standalone DACs  1% of that.  I’m not including HT receivers and processors here, just the rarity of multichannel.