How come accents in movies used to be so bad, but now are so good?

A couple generations ago, an American trying a ney accent would result in an atrocity such as Van in Mary Poppins, and the only Brit who could sound American was Peter Sellers. These days, the most casual moviegoer would have no idea that Cate Blanchett is Australian, Ewam McGregor is Scottish, Johnny Depp is American, Hugh Laurie is English or that Colin Ferrell is Irish. Why the change?