Not to quibble, but...why not?

...those were humans who ate human brains in a society where there existed an unexplained fatal illness (previously called "kuru"). There was no proof (or even real evidence) that the brains transmitted the disease, particularly since "prions" were not even known to exist until the '80s, and not known to be connected with BFE or its human counterpart for several years after that.

The kuru situation - because the infectious agent was unknown and unsuspected - seemed to be far removed from people eating beef from cattle that ate grain supplemented with protein from animals that appeared to be perfectly healthy.

It all makes sense in hindsight, but at the time - when it was not even suspected that humans could contract the cattle disease - how could they know any of this?

How many similar surprises await us in the future?

This might be good place to mention the "Law of Unintended Consequences."