Why did they change the food pyramid?
A new generation of scientists and better studies determined a change in the so-called USDA Food Pyramid.
Wikipedia calls the USDA's original food pyramid from 1992 as "outdated".
The old recommendations have mistakenly emphasized a large carbohydrates group (listed with 6-12 servings), instead of the healthier vegetables group (which are listed with only 3-5 servings).
Moreover, most people understood at that time that it was OK to eat large amounts of white bread & cereal, white rice, white pasta, white table sugar ("simple carbs") -- when studies started to show that they are detrimental to health.
Today we know that a diet based on "simple", over-processed carbs, is deficient of essential nutrients and fiber -- and it is leading to an epidemic of diabetes, heart disease, metabolic syndrome, obesity and worse.
A change from "simple" (or processed), to "complex" (or natural) carbs would encourage better nutrition -- the way it was hundreds of years ago.
The US National Library of Medicine (which is part of the National Institutes of Health) state today that "Complex carbohydrates provide vitamins, minerals and fiber": https://medlineplus.gov/ency/imagepages/19529.htm
The other mistake in 1992 was a lack of dissociation between healthy and unhealthy oils (and fats). Studies show today that trans-fats, over-processed or "refined" oils, hydrogenated and semi-hydrogenated oils (and fats) are unhealthy.
See: The British Medical Journal. "Banning trans fats would save lives, say doctors." ScienceDaily, 16 April 2010.
"The U.S. Food and Drug Administration (FDA) told food manufacturers to stop using partially hydrogenated oils (PHOs), the major source of artificial trans fats in processed foods ranging from nondairy creamers, to baked goods, margarine, and microwave popcorn".
The move, according to the FDA, “is expected to reduce coronary heart disease and prevent thousands of fatal heart attacks every year.”
[Source: "4 Things You Should Know About the FDA’s Trans Fat Ban", June 19, 2015. https://civileats.com/2015/06/19/4-things-you-should-know-about-fdas-ban-on-trans-fats/ ]