In order to answer that question you have to go a ways back to find out why circumcision of non-jew boys started in the US. It was a combination of three things all coming together at roughly the same period of time in the late 1930's to early post-WWII years.
Cultural hostility towards masturbation and OBGYN's thought it helped prevent STIs, removed possibility of congenital phimosis (which is true, although ridiculous to subject all baby boys to this before it even comes up as an issue). Combined with this, the gom clamp came out around the same time which made it possible for non-surgeons to perform circumcision on infants.
The US military circumcised a huge percentage of soldiers because they thought it would reduce risk of STI/STDs.
Comprehensive healthcare insurance offered by companies in the post war boom era covered the procedure. So you have circumcised fathers having kids, obgyn's who were selling the idea to them hard, and insurance companies covering it.
Why it stuck around after all of the theories about it were debunked is another question.
