Why aren’t BNC jacks common?

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.  

By long ago, I meant from the 1970's. Dick Sequerra, George Kaye, John Curl, Bob Fulton, Dick Marsh in my experiences with them (in and outside audio) all agreed and used them on their personal systems or research designs.

I love BNC terminators, especially with good quality cabling.

From the original post, my comment though is that I am aware of 75 Ω and 50 Ω impedance for BNC (50 Ω normally reserved for clock signals from devices like the Mutec Ref10 Series) for Word Clocks and Master Clocks, whilst the default is usually 75 Ω.

110 Ω is usually reserved for AES/EBU digital cables - another fav of mine.

My T+A DAC 200 has support for these (but no clocking) as do many others like the Holo May KTE.

Then devices like the Hegel Viking CD player also has 75 Ω BNC output.

A special case is the Chord Hugo TT2 / M-scaler which have DUAL BNC for their special upsampling to 768KHz instead of the standard limitations of 384KHz. Won't work with anything else!

I won't delve into any preference for cable brands - there are many and it's really personal preference and of course one's wallet!

I am a career electrophysiologist (neurophysiologist).  BNC cables have been the standard cable in electrophysiology labs since at least the ’70s.  Simply put, they’re reliable, flexible, quiet, easy to make, and inexpensive.  RG58 cable and BNC connectors - done.