Most babies born in the US in the 1950s and 1960s were formula-fed. It was what most mothers did then. It regained popularity when the health benefits became more widely known, but the social changes and more relaxed attitude about discussing previously taboo subjects were a big part of it, too.

From Wikipedia: “By the 1950s, the predominant attitude to breastfeeding was that it was something practiced by the uneducated and those of lower classes. The practice was considered old-fashioned and “a little disgusting” for those who could not afford infant formula and discouraged by medical practitioners and media of the time.”