My understanding is that it was a loss of institutional knowledge that came from the passage of time and from arrogance. The engineers who knew why the additive was required were fired or ignored or both. Could also be a lack of documentation?
Edit: The article has a chart that says just one additive would have improved things, but still left the water 3.5x more corrosive than the original water source: https://www.materialsperformance.com/uploads/images/Articles...