At first, English speakers in the colonies and England did use a rhotic accent. But after the Revolutionary War, upper-class and upper-middle-class citizens in England began using non-rhotic ...

Around the turn of the 18th to 19th century, not long after the Revolution, non-rhotic speech took off in southern England, especially among the upper and upper-middle classes.

If you say, I pahked my cah in Hahvahd Yahd, like some vaudeville version of a Boston accent, you are non-rhotic. If you say, I parked my car in Harvard Yard, you are being rhotic.

For example, the New Yorker accent is one of the most visible regional accents in American culture, ... She used the example of being rhotic vs. non-rhotic — in other words, ...

American speakers have rhotic accents, or pronounce the r’s in words like “park” or “harp.” British speakers have non-rhotic accents, so the words sound more like “pahk” or “hahp.” ...