It now seems quite possible that the principal reason behind this catastrophic failure was the designer's decision, 50 years ago, to use steel tie bars clad in concrete as the bridge's main tension members.
This was essentially a cable stayed bridge design in which the steel tension cables were wrapped in concrete instead of paint (or a similar corrosion resisting medium). The concrete wrapping provided no structural benefit to the bridge and instead went on to prevent the ready inspection and maintenance of the main steel strength members it enclosed.
Concrete is excellent for compressive loads but useless in tension. Steel is excellent for tensile and compressive loads but needs a protective coating to prevent corrosion.
Also it seems that there was also no redundancy in the structure - a single member failure could lead to total collapse.
These are significant design failings in my view.