Before the age of electronics and film, when "Show Business" meant touring live performers, they had to develop a dialect and performance/speaking style that would be understandable by the largest number of people. From Maine to Washington State to Southern California to Florida,and every place in between.
Thus was born the "Transatlantic/Standard English" dialect.
When broadcasting and film came along they adopted it as well, because it was understandable by the most people.
