It has to do with the old-timey thoughts that it's cleaner, and it's just become tradition here. The HIV issue is really only important in sub-Saharan Africa where it's still spreading rapidly, not so much in the US where there's tons of education and available barrier methods, as well as preventative meds for high risk populations.
Circumcision is painful for the baby, and I know many American men who are very sad they are not "whole" and experienced nerve/sensation issues as a result of it. One friend who is an OB/GYN hates doing them, and if he had his way would never have to cut another baby but his practice would not go for that - they earn a lot of money. If I manage to have biological children and one is a boy, they would not be snipped!