Indeed. I find this so strange about the USA. For most there is no religious reason for circumcision so why has it become so prevalent? Very weird for sure..
You could write book after book on the subject probably.
But a very short and concise answer would be that it was most likely a cross of worries (well, more like hysterical scares) about the adverse effects of masturbation - which in the 19th century was thought to be the cause of many kinds of illnesses and debilitating diseases, running the whole gauntlet from TBC to feeble-mindedness and anything else quacks and religious preachers and teachers and nutcase doctors (think J. H. Kellogg for instance) could think up.
If you couple that to an easy to do, fast and reliable surgical procedure, you also get a lot of doctors making easy money off of worried and concerned parents.
So, make parents scared their male kids will end up scarred for life or prematurely dead or devastatingly ill, then get them to pay for a small surgery and then hope for the ball to get rolling that social conformity will take care of the rest.
The biggest impulse for almost universal circumcision in the US were also the two 20th century world wars. Men were drafted into mass armies and doctors were convinced that for reasons of in-the-field hygiene, all recruits should be circumcised, whether they liked it or not. In some branches of the services it was mandatory and in others it was not even an elective procedure. A commanding (medical) officer could order it done and it was also often performed surreptitiously, when a soldier was undergoing other surgery.
So, by the late 1940s circumcision had become neigh universal in the US through a combination of scaremongering quackery, an opportunity for docs to make some easy money with little risk and the desires of the armed forces to keep genital health problems to a minimum.
Of course it is more complex than this alone, but it's a good framework to work from.