Why aren’t BNC jacks common?

The question also could be posed in the past tense as, "Why wasn't BNC more common before for audiophiles?"

Today, between (a) streaming over ethernet or Wi-Fi and (b) and the portion of the community that has taken up upscaling using ethernet or I2S via HDMI cables, I'd be surprised to see BNC much longer in audiophile gear. 

The exception to that might be any product that sells a fair portion of its volume in Japan, where BNC was more common. 

Older Naim preamps used to use a mix of BNCs (for phono inputs), and DIN connectors for line level inputs and outputs.  Not an XLR or RCA in sight!

@carousel 

Not true. BNC connectors have been the standard for 50 ohm cables in electronic instrumentation for many, many decades. Certainly more reliable than the RCA ones.

I am all for adopting BNC as the connector of choice in audio systems (XLR is also good for balanced cables. Reluctance to adopt new ideas has hampered its widespread consumer use and will undoubtedly continue to do so.

Indeed, I am a big fan of XLR. I have zero Cinch-RCA in my systems. Only balanced.

LONG ago I ditched rca's and converted all my equipment to bnc.

Yep, the so called cognoscenti at Audio Research would and did howl I'd violated their warranty... but then again I as an licensed avionics repair station engineer I rarely set anything back to them anyway! So YES BNC should be the  standard. By the way most rca connections are very close to 50 ohm imp.