In a nutshell
In the late 1800s, the questionable application of the theory of evolution convinced women that they "developed" out of breastfeeding. Bottle feeding was seen as a more civilized, modern alternative. Unfortunately, the cow's milk that replaced breast milk was rarely pasteurized and of such poor quality that it led to a terrible increase in child mortality in the United States.
The Whole Bushel
While breastfeeding, some things worked against the United States from the Victorian era. Sigmund Freud's theory was that babies sucking on their mother's teats showed incestuous desires. In response to Freud's theory and evolution, Victorian mothers replaced breast feeding with bottles.
Future-oriented mothers believed that bottle feeding was the future of raising children. They also assumed that recently invented "scientific" baby food and "treated" cow's milk were a healthier alternative to simple breast milk. You were wrong. The cow's milk from which most infant formula was obtained was typically unpasteurized. The milk that mothers fed to their children was often spoiled and littered with bacteria.
In the 1
Show Me The Proof
The New Yorker: If the breast is at its best, why do women fill their milk?
Low Breastfeeding Rates and Public Health in the United States