Why aren’t BNC jacks common?

I have owned multiple high'ish end DACs and very few of them had BNC connectors.  The adaptors may "work" to conduct a signal but in the audio world where I constantly read about how "everything matters" it perplexes me how something like this that is so simple to get right doesn't adhere to some sort of standard. FWIW, AI says,

BNC/RCA adaptors on a 75-ohm digital cable technically introduce an impedance mismatch, as BNC/RCA connectors are not designed for true 75-ohm impedance. While the cable itself remains 75-ohm, RCA connectors are generally around 50 ohms or non-impedance-controlled.  Therefore, the connector transition can create small signal reflections. However, at typical S/PDIF frequencies and short cable lengths, this mismatch is often negligible.

I have gone back to using USB connectors into both of the DACs in my main system, because IMO they sound better.

This has baffled me as well, BNC is such a superior connection. I don’t use SPDIF in my system anymore, but I always had either BNC or AES as a requirement when I did.

@zlone in my system I run two digital components into my DAC, one is a streaming transport and the other is a CD transport. Both have AES outputs and RCA based SPDIF outputs plus toslink. The streamer also has a usb output. My DAC has an aes input, rca/SPDIF input, BNC/SPDIF input, usb input and toslink. I am not a huge fan of usb or toslink, so I am left with SPDIF and aes. My streamer feeds the dac via aes and cd feeds the dac with a cable that is rca on one end and BNC on the other end. 

My streamer feeds the dac via aes and cd feeds the dac with a cable that is rca on one end and BNC on the other end

That is unfortunate that you have to use an adapter. I never liked USB that much, but I eventually came around to it as my equipment and cabling progressed. 

I see that you have a Bricasti DAC. I recently acquired an M3 with the network card and have switched to using that input as a Roon endpoint, it far exceeds the performance of the other inputs on the DAC. 

The audio industry wants to sell you expensive cables when the real problem has always been the connectors. Spend the money on DartZeel and be done with it.