I have a pet theory that half the reason that the public bought the overly simplistic theory that fats are bad is due to the collision in English between the word "fat" as a type of substance, and the common use of the word "fat" as a pejorative adjective.
This has even affected some of my family members for whom English is a second language, but who learned both meanings of the word simultaneously in the 80s in the US. Some of them can't separate the concepts, no matter how hard I try to explain to them that they are mostly unrelated.
I think another reason people jumped all over it was vegetarianism - less fat (especially saturated) equates to eating less meat. Whether you are a vegetarian for environmental (raising meat requires much more resources) or for ethical (PETA) you would want to latch on to the low fat thing because it agreed with you.
This has even affected some of my family members for whom English is a second language, but who learned both meanings of the word simultaneously in the 80s in the US. Some of them can't separate the concepts, no matter how hard I try to explain to them that they are mostly unrelated.
EDIT: wording