There are WAY more opportunities for directors across the UK. It's a smaller country and most producers don't have to go way out of the way to view an up-and-comer to file their talent away for later use (I'm sure, say, Kansas City might have some really good directors, but whoever gets to really see them work to get a reputation). There are often tours of established shows with "new" directors reshaping productions. London itself has many more "professional" opportunities, a lot of West End, fringe and off West End productions don't cost a lot to produce and/or are subsidized. And a lot of these directors get to cut their teeth on American plays and musicals, allowing them to experiment in ways American audiences don't necessarily allow. I saw the recent Doll's House, & Juliet and upcoming Cabaret revival directors direct fringe and off West End long before they became star directors. Chicago gave us a bunch of talent for the same reason, as New York non-Broadway companies rarely do revivals, and working on classically written pieces, I'm sure, really help with the development of a director. British directors get to experiment, they get to fail, they get to find their own voice. That's very important.