It was more of an advertising thing, from what little I understand. Products sold better if they had "0% Fat!" on the label because some people we're confused about healthy fats. I'd have to ask @corinnestokes for more details, but from what I remember reading, the cultural idea that fats were unhealthy did spread to a number of people (which is why advertisers responded as they did, to get that purchasing power) which, ultimately, hurt people's health. It also relates to the cholesterol discussion which pushed out healthly fats as well. This isn't my area of understanding, so I'm worse than an armchair quarter back here.