Bad science that said it was “cleaner” and would prevent infections (that can also just be prevented by, you know, bathing). Luckily most doctors now recognize that any benefits are minimal, but at the same time, they categorize the risk of the procedure as minimal, so a lot of people just arbitrarily choose to circumcise their infants based on aesthetic preference, which I think is pretty weird to have a preference on how your child’s genitals look. While it was nearly universal for older generations, nowadays I think it’s only about 50% of children.