Why so few devices with BNC's??

It’s "Bayonet Neill–Concelman". There’s also the threaded version TNC. Paul Neill also invented the "N" connector and Carl Concelman invented the type "C" connector. The type C resembles an oversized BNC.

BNC connectors are available in 50-ohm and 75-ohm types with the 50-ohm being much more common. They’re physically interchangeable without damage, the primary difference being the lack of an inner plastic ring on the 75-ohm version.

This thread is about digital connections! 

Sorry I didn't notice that until now. The answer is the same though, audio manufacturers are comfortable with RCA connectors and they cost less and do the job. 

A RCA connector is not a particularly good analog connector, even though it can be made at many price points. It is not a digital connector. The impedance of a cable, and its associated connector is determined almost solely by the diameters and spacing of conductors. The materials, and even the permeability of dielectrics is a minor influence. The physical size of the RCA connector is such that it cannot be made to be 75 ohms, no matter how much money you throw at it. When used as a digital connector it will cause a singularity in the flow and a reflection. This is why you see so much written about a RCA digital cable needing to be a certain length. I am not aware of length being a problem for AES/EBU and true 75 ohm cables and connectors. In my experience, for cables meeting design norms, shorter was a little better, and these cables do not need to be expensive. (This does not preclude that golden eared listeners may prefer more expensive cables.)

It is lamentable that bnc is not the defacto norm for 75 ohm digital cables as a true 75 ohm cable can be built with a bnc cable. 
 

Years back on my first outboard DAC I asked the manufacturer if they could use a BNC connector in place of the RCA s/pdif input. They  were happy to oblige. 

For Muse electronics, designed by Dennis Halverson, all digital and video connectors were BNC.