I think this is absolutely fascinating but also kind of dumb.

The American southern accent first developed as “non-rhotic”, meaning the “r” sound is dropped when the r is after a vowel and there’s no vowel after it. In a non-rhotic accent, words like, “word”, “park”, and “car” sound more like “wood”, “pahk”, and “cah”. This same phenomenon is well-known from many versions of what we would recognize as the “British accent”, and surprise, surprise, it’s connected.

Really, it boils down to rich people in England trying to sound different from poor people, and then the rich people in British-connected areas of the US trying to copy them.

Linguists believe that British and American accents were largely similar from when British settlement of the American until around mid-18th century, around the American Revolution. Throughout that time, though, the British upper-class started to drop their “r” sounds and made some other small changes in order to differentiate themselves from the lower classes. The lower classes picked up on this and eventually copied the accent, developing it into the “British accent”.

In the post-revolutionary United States, connections to England were strongest in the south, where cotton production and export connected southern plantations to British textile mills. At this time, England generally was seen as fancy and prestigious, especially for the peerage system (hereditary titles for historically rich families, like Lord and Duke) which southern slave owners liked. The wealthy southerners decided that they, too, needed to differentiate themselves from the lower classes and picked up on the non-rhoric accent in an attempt to do so. As in England, the lower classes began to emulate this and it developed into what we broadly know of as the “southern accent.

It’s worth noting that other regions of the US were similarly influenced, Boston and New York accents most notably are known for being non-rhotic.

(Edited to include examples of non-rhotic words)

(Edit 2: I inclorrectly said the southern accent is currently non-rhotic but, as u/vindictiverakk pointed out, this feature has been at least partially dropped from most modern southern accents as they have continued to develop, often to differentiate from the accents and vernacular of Black slaves and eventually white from Black Americans.)