Talking about fat seems to have been more attractive because it's more "obvious" (see also: how long we were stuck on wrong ideas about dietary cholesterol, and still seem to be about sodium). Also because there's more chemical variety in fats to argue about. You can have complete fats vs. fatty acids separated from a triglyceride backbone; you can have saturated, monounsaturated and polyunsaturated fatty acids; and each point of "unsaturation" (where there is a double bond between carbons in the "backbone" of the molecule) allows for cis/trans isomerism. With sugar you basically only have glucose and fructose, and chains thereof, to think about; and that also covers starches automatically.