The Trans-Atlantic Accent (Or Why People In Old-Timey Movies Talk That Way)

I think that the dominant motivation was a desire to emulate if not ape supposedly more sophisticated British pronunciation. My sense is that it was taught and modeled in Northeastern private and New England boarding schools, which tended to appropriate British elements more generally. The accent fell out of favor after World War II when Britain ceased to be the cultural, and certainly economic and military center, of the English-speaking world and focus shifted to metropoles such as New York and Los Angeles.