"Why couldn’t the regulators in Brunel's day have carved out an open-ended “unless otherwise demonstrated” around the use of iron structural members?"
The answer is partly in your statement: "regulation is, in isolation, neutral." Looking around at regulatory/ bureaucratic bodies, "in isolation" is rarely (never?) achieved. Satisfying ambiguous "unless otherwise demonstrated"-esque clauses that are enforced by unelected paper pushers is very much part of Brunel's concern.