The most recent time it gained popularity (in the US) was because doctors and psychiatrists started saying that masturbating caused serious harm. It was advocated as a punishment for male masturbation. Source.
Even though (most) people know better now, many adults choose to circumcise their children for any number of reasons.
-Fear it will hinder his ability to fit in whem changing/ getting a girlfriend.
-Because they want him to "match" the father so he won't feel strange or different.
-They're misinformed about the benefits/ draw backs.
-A doctor suggests them it and they're totally uninformed so they defer to the doctor's authority.
-The exceedingly rare cases where it is medically beneficial because of issues with the penis/ foreskin.