Two beliefs became entrenched:

1. that lead is natural to the human body, and

2. that a poisoning threshold for lead existed

Robert Kehoe, working for GM, was the chief advocate for leaded gasoline, and really the only person/lab doing research on lead until Clair Patterson stumbled into it while measuring the age of the earth. [0,1]

A modern equivalent might be if Facebook was the only organization researching social media's impact on society, while being able to set the paradigm/assumptions about said safety for half a century.

So even when Patterson's research was published in 1965, it took time to change the paradigm, and more time to phase out lead's use.

Should anyone want to read a narrative about the intertwined lives of Midgley, Patterson, Kehoe and lead, then this Mental Floss article is a good read. [2]

[0] https://en.wikipedia.org/wiki/Robert_A._Kehoe

[1] https://en.wikipedia.org/wiki/Clair_Cameron_Patterson#Campai...

[2] https://www.mentalfloss.com/article/94569/clair-patterson-sc...