Investigators couldn't find the design calculations so it was unclear how the flaws made it on to to bridge. Once they made it onto the completed bridge there was little chance they would be noticed by inspectors.
Aren't there archives? Were the design calculations lost or hidden.
There have been contracts where a low quote is offered to win a tender, with the contractor having to cut corners to make money.
The design flaws would not have been noticed by inspectors - unless the bridge collapsed
By now most of the contractors would have died of old old age, or be in congress.