You typically don't have a design engineer actively monitoring safety features that are permitted by the regulator, built and maintained to standard. In a dynamic environment where the situation changes regularly there is a little more defense for catching that type of thing. In a static environment like fixed plant it could well be they just weren't ready for that sort of change.

That style of hazard, where existing controls passed through a rigorous system but become insufficient with time, are really insidious. It is certainly easy to understand why they would have missed it; even though it isn't an acceptable outcome. Especially given that communication in the 70s/80s involved a lot more talking to people and reading the right journals.