It’s hard to believe that the pronunciation of a single letter has been the subject of so much study, but it’s true. The evolution of the various pronunciations of the letter “r” has actually been documented and is surprisingly interesting. The story begins well before the American Revolution (in the 1770s) when the speech of most British citizens and most British colonists in America was very similar. The speech of the English upper classes at the time, however, was notably different, and one of the main differences had to do with the pronunciation of the letter ‘r’. To put it simply, the upper class in England made it a habit of not pronouncing the letter ‘r’ in the middle or at the end of words (so that “far” sounds more like “fa”). As the social class structure of English society changed and became more fluid, it became easier for common folk to rise up into the middle and upper classes. As people rose up through the classes, learning to speak with a “proper upper class accent” became important to them. This naturally involved dropping the “r” out of their speech.
People in American East Coast cities like Boston, especially the upper classes, had very tight relations with England in the 1700s and 1800s. It was only natural then that they imitated the upper class English when they spoke, hence the dropped “r” in the Boston accent. The Boston/New England accent never spread to other parts of the United States because as time passed, however, because immigrants from other countries besides England began flooding into the United States,and the upper classes of new American cities in the Midwest were far less concerned with speaking like English nobility.