One presumes Americans took it from the Jews and not from the Muslims, but as to why, I don't think there's a single explanation. Wikipedia suggests that it had to do with somewhat primitive views on germs in the early 1900s on the one hand, and on the other hand with preventing masturbation. Though it also says, somewhat surprisingly, to me at least, that only 60% of American male babies are circumcized today, and the sources seem to disagree significantly on how high the percentage was at the time when it was most prevalent, from 70% to over 90%.

In Australia, it was apparently quite prevalent in generations past, but is falling quickly now. In the UK, it never was dominant but was more prevalent in the past than it is now.