Epidemiological studies done a couple of decades ago indicated that
populations with high fat intake had a high degree of obesity and a high
degree of heart disease.  Research showed that lowering fat intake
frequently lowered total cholesterol.  Thus the conclusion was that high
fat intake equated to obesity, high cholesterol, and heart disease.  Also
some people, like Pritikin et. al. showed tremendous improvement in many
patitents when put on low-fat diets.

The problem is that time has educated us.  Some people do absolutely thrive
on low-fat diets. Also, some individuals lose weight like bandits on
low-fat diet (I've encountered a few).  So we've long had these these
individuals wandering around with impressive testimony.

However the long-term results show that lowering fat intake does nothing to
reduce mortality from heart disease or cancer or anything else.  In fact
what few studies have been done on this have shown that low-fat diets
actually very slightly increase mortality, though so far the increase is so
small it's not concisdered significant.  Further, the majority of people on
low-fat diets either do not lose weight, or lose only small amounts of
weight which tends to return in time anyway.  For a majority of people,
low-fat diets are a complete waste of time.  Worse, there is now
considerable evidence that such diets are actually deadly in the long run,
raising the risk of cancer, diabetes, and heart disease for some people.

However, it takes large health organizations a long time to catch up on a
lot of these things.  There's also a political agenda among vegetarians and
companies and individuals who produce grains and vegetable oils and such.