Chicken and egg story all over again. Which came first? In this case it was CD players and transports. If very few have BNC outputs then DAC builders don't see any need to include a BNC input.
I am not arguing which interfaces are better just that there are so many digital interfaces those that were not common became even less common.
Interface connectors I can think of: RCA, AES/EBU (XLR-style connectors, USB, Toslink, Glass fiber but not Toslink (ATT?), i2S, BNC.