I routinely work in the nanovolt ranges, the femtoamp
ranges in some very robust environments, also often
140 dB dynamic ranges, so noise pickup is a BIG issue.

There's another issue we haven't discussed, and that's electrochemical
reactions. BNC connectors aren't well-sealed against the environment. You
don't "stuff" getting into the connector and corroding the connection.

An XLR connector offers no only a balanced line, but the contacts have
greater surface area, and you can seal off the connector will silicone.