Margarine and butter both sort under "fat" in old dietary guidelines. The reason schools (or anyone) preferred the former was because of cost.

I bet government food recommendations was just the starting point, and concerns about cost and getting kids to actually eat it would have transformed any plan into something similar. Calories processed into pure forms (sugar and fat) are cheap. Grains are super cheap, which was a big part of why they were the backbone of dietary recommendations for so long - no use instructing people to eat something they can't afford.