Why so few devices with BNC's??

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.

BNC is an acronym for British Nut Connector.  It is most often encountered in measurement equipment and some older hi-fi gear from the likes of Dayton-Wright in Canada.  With the well entrenched RCA Connector it is difficult to usurp.

It is dependable connector that has the advantage of making the ground prior to the positive connection.

"BNC is an acronym for British Nut Connector"

Incorrect.  The acronym is for Bayonet Neill–Concelman.