It’s hard to imagine, but in traditional english language, in British isles as well as in british colonies in America, sound, which was marked with letter “r” once was pronunced clearly, not as it is now, when sound “r” is guttural. These days british instead of “hard winter”, like with a hot potato in their mouths, would say “hahd wintuh”.
English language dialect without clear “r” pronunciation formed in American Revolution times, in South England – London and it’s districts – high societal circles. This dialect formed, because people from lower social circles, but after Industrial revolution, the ones british people who became rich, deliberately tried somehow to distance themselves and separate from lower social class. In other words, “new british” started to prim and practice soft, guttural “r” pronunciation, to demonstrate new, hihger social class status.
This London dialect became privileged in new specialists – orthoepy and rhetoric teachers, whose services was very expensive – ranks.
So this prig like new language manner soon became as a language standart. Officially it was called „Received Pronunciation“, and it took not much time to spread all over Britain. However, in North england, Scotland and Irish people for a long time saved their traditional, clear sound “r” pronunciation.
Most of the american accents has saved this clear ¨r¨ pronunciation. But there was some exceptions, in New York and Boston became popular new sound “r” pronunciation, because these cities after Revolution war was in strong british elite influence zone.