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

Good DACs take USB and galvanically isolate it from the input section, thus eliminating most of the electrical noise that it might otherwise carry, hence USB can (in all but the cheap stuff) be a very good input, even from terribly electrically noisy computers.  In the last few years I2S has made a splash because of companies like PS Audio developing their own "standard" for the pin outs and many other companies have unofficially adopted that same "standard" so they can play well together without the end users being a geek.

I2S has the "claim to fame" that it separates out the clock signal from the data stream and in theory that can allow less jitter and noise.

Note that I2S existed inside digital devices long ago well before HDMI came along. They are just implementing I2S via an HDMI connector now because those were available, relatively cheap, and semi understood by the masses as a "cool connector".

None of my devices so far have an I2S connection so I use USB (instead of coax or Toslink optical). Toslink optical might have been a great digital connection if glass were used instead of plastic and if they had upgraded it to be more robust in bandwidth, but it never was, hence the limitations make it the "worse" connection you can use at least if you are trying anything better than CD Redbook quality bit depths and sample rates.

I’ll take it on word of mouth that in $$$ systems, I2S can often sound better. Overall, it isn’t something I’d worry about. The DAC you choose, speakers, amp, and your room are liable to make a bigger difference in sound quality.

Note, it may be years before "slow as molasses" industry governing boards like the IEEE and their audio steering committees to create or adopt a "real" I2S standard. 

Most streamers don’t have I2S outputs hdmi 

If you have a DDC Reclocker then they have I2S outputs  

the only brand I know that sell true I2S  audio cables without the video wires is the Excellent Tubulus Audio cables.

@2psyop "USB was for data not audio." Sending audio DATA over USB *is* sending data, zero and ones. Digital data transmission does not depend on what type of data it is. Data is data from a transmission perspective.

HDMI isn't really the correct terminology for audio only use. Yes, the connectors are HDMI, the cable itself needs only be I2S, this means only 8 pins actually wired, the rest are superfluous for audio only. Plenty of nice audio only I2s cables out there today, Tubulus and Audiolund are a couple manufacturers.

I2S has a couple advantages over usb in that data and clock runs are segregated, usb has to extract data from clock run. The other advantage is I2S is native signal path within dacs, usb inputs converted to I2S. Having said that manufacturers have gone to great lengths optimizing usb at both the streamer and dac, so usb can be very nice interface these days. Theoretically, I2S imputs on dacs shouldn't require as much work as usb, still clock and power supply to that clock should be optimized. I2S is virtually unknown output on streamers, and if included not likely to be optimized. So now we come to the greatest liability of I2S, that being not designed to be sent over cable, clock should be located in closest proximity to I2S circuit within dac. For this reason best to limit I2S cable length to .5M or shorter, using such a short cable could be problematic for many due to equipment positioning issues.

And then we have the question of optimization in the implementation of usb and I2S, such that a well implemented usb would likely better less than optimized I2S. Top of pyramid would be quality implemented OXCO clocking on either of these interfaces, this mean quality clock and power supply to that clock. Next would be TXCO, then Femto, this hiearchy not set in stone, therefore, quality implemented TXCO or even Femto could better less than optimal OXCO.

Bottom line, so many variables, individual setups could favor usb or I2S. In my case I've come to the determination I2S superior to usb since I have both well implemented usb and I2S to choose from, I2S beats out usb. I also maintain I2S inherently superior to usb based on reasoning explained above, this as long as I2S cable length .5M or less.

'Experts' have a variety of opinions in this matter, some refuse to offer I2S dac inputs based on a particular mindset, others offer them based on another set of reasoning. No one I'm aware of offers optimal I2S input in dac, OXCO clocks with high quality power supplies simply take up too much real estate to offer in dacs. Optimal I2S rarely if ever offered on streamers either, usb seems to be their de facto output, sometimes network. This is where ddc come in, they take what is hopefully optimized usb out of streamer, convert to quality I2S that is synced or slaved to dac, this means one is using clock and power supply from ddc vs dac internal clock and power supply. There are a small number of custom streamers such as mine able to directly output well implemented I2S to dac, this means bypassing usb altogether, this may be superior solution.

Finally we have the proprietary interfaces like the ones in Auralic, Taiko,etc. based on reviews these surpass both usb and I2S.

Here I am quoting from a respected DAC designer:

OK...see if this makes any sense to you: if clocking gets corrupted with a single channel traveling on one wire then how would it make any sense to attempt to coordinate three separate clocks on three separate wires?

It makes no sense.

If I2S was actually better they would be using it in recording studios and they most certainly do not.

If I2S was actually better then nearly every company in the audiophile industry would be promoting it and they most certainly do not.

There are a small group of Chi-Fi manufactures who started promoting I2S and the audio-fools bought into it hook-line-and-sinker.

If I2S sounds better in a specific DAC it is only because the other digital inputs on that DAC are lacking, not because I2S is inherently better.

The above excerpt is from pg 7 of this excellent thread.