I think it's getting less common now for newborns. I'm circumcised, and I don't have any problem with it... Everything works fine. My wife says she doesn't like guys uncut, so that's a good reason right there! I also have a friend who had to get circumcised at age 20 because he kept having some kind of a circulation problem, and that is probably WAY worse than getting it done as an infant... Imagine what would happen the first couple of mornings after getting it done as an adult; OUCH!
As far as how it got started; it's probably cleanliness. As pointed out early in this thread, most of us take plenty of showers, and that's a whole lot different from 100 years ago when a whole family shared 1 bath tub full of water once a week.
Charles