Epidemiology is the study of disease patterns, in essence. The problem is that by the 40s and 50s things like smallpox, polio, etc were pretty well understood. For smallpox the problem was the effort it would take to eliminate wild smallpox, getting vaccinations out to every corner of the world - it was known what was needed, the how was another question. With polio there was a very extensive effort to find a vaccine, you could move that up a few years but...The connection between smoking and cancer, specifically and lung cancer, had been brought out in the 40s. The tobacco industry fought this tooth and nail for decades. Pesticides, air pollution, lead paint and leaded gasoline, on and on.
It wasn't that the medical specialists/epidemiologists hadn't developed evidence that this, that, or the next thing was bad for population health. It was that those people who made money off of this, that, and the next thing fought hard and bought politicians and tried to buy public attitudes for a long time. "It's only a theory", "data is incomplete", "more studies are needed", "doing {this} will cost the average Joe a lot of money", "you are infringing on peoples' freedoms". All this sound familiar (like insert 'climate change').
BTW epidemiology does not necessarily interact with a better system of delivering health care, that's a separate issue.