Wow. big question. Read the name chapter in Freakonomics
I think more and more boys names become 'unusable' (or considered risky) as people (Americans first) start using them for girls. Then parents have to start looking for alternatives, like say Harvey which would just have been a sur name 20 years ago.
Some people say that upperclasses revere their pasts, and so they are drawn to old names like William and Peter.
WOrking classes or more optimistic about the future than glorifying their pasts, so they are slightly more drawn to modern-sounding names. Like Kody for example!
I read that somewhere btw, not my observation! Possibly in freakonomics, not sure.
Americans now that they have all merged and blended into a big melting pot can afford to think about their pasts a bit more. So they dig up names which sound Irish/Scottish to their ear, but in fact conform to American trends.
eg, Aidan cos we're of Irish descent, but bang a K on the front, sounds 'cooler' and you have Kaiden....
I think I read that in 'the name wizard'.