It has to do with the way we've been developing acting and comedic talent in the States the last few decades. Gone is the American (many times character) actor who was forced to develop well rounded acting talent through numerous bit parts in movies, tv shows, and theater, and classes. If you look at the best American actors both back then, and now, that is how they were brought up. Now they just seem to grab some pretty kid, plop him in front of the screen for some very shruggable film, and there is not a huge eye toward the long haul.
Meanwhile, the Brits, and foreign actors are aged and well trained by the time the get here. They are coming up in they way Americans used to.