KGS is still huge, but OGS has been consistently adding feature after feature for years. At some point, my impression is that most English people just realized it has a nicer interface and better analysis and cheat-detecting tools. The main downside is a lack of high-dan players.
(I don't have data to back this up, just my experience as a roughly 1-dan player who hangs out with a lot of U.S. Go folks)