There’s actually a specific answer to this question. Edward Bernays, the pioneer of modern marketing strategies. Was hired by a bacon producer in the 1920s to promote their product. Up until then, bacon was consumed mostly at dinner, and the typical breakfast consisted of toast and coffee. Bernays saw an opportunity to expand the bacon market into the breakfast niche.
He got a doctor to write a report saying that a substantial breakfast was better for you, then got several other doctors to endorse this view in a survey. He used this survey as the centerpiece of a publicity campaign promoting bacon together with eggs (also never a common breakfast food) as the ideal breakfast.
Like almost all of Bernays’ other promotions, the campaign was wildly successful and changed the habits of the consuming public.