I don't think you can easily separate the two. Things might start as a marketing campaign but then catch on in general culture. In modern culture, this would be true of bacon. Bacon was falling out of favor in the US in the 1980s as people were beginning to become heart and weight conscious. But then there was a massive marketing push and now bacon is everywhere even on things like donuts. People think bacon is the most wondrous food/flavor and probably don't associate their obsession with a marketing campaign.

I don't think it's because of the milk lobby (which didn't exist until early 1900s according to other posts) because I once browsed a cookbook from the 1800s and noticed that many of the recipes called for an amazing quantity of butter. The recipe for bread, which made two loaves at once, called for 1 pound of butter. Pound Cake also uses a pound of butter.

People ate less meat/poultry back then. Dairy and eggs were the most common sources of animal fats and protein.

I mean it is cheap while being nutrient and calorie dense. It makes sense to put on a relief menu lobby or no