Whether George Bernard Shaw actually uttered this much quoted phrase or not is of little importance. What is indisputable is that there are some profound differences between British English and that spoken by our cousins across the Pond, that go far deeper than the superficial (but still hilarious) embarrassment of ‘pants’ for trousers and ‘panty hose’ for tights.
Rhoticism
Just like us, the Americans have a standard ‘no accent’ version of their English, which is perceived as neutral and desirable in some official quarters. Here, we have RP – standard Received Pronunciation – or BBC English, if you will. In the US, it’s called GA – General American, ‘newscaster accent’ or Network English. If we compare these two standard ‘codes’, the most obvious divergence is rhoticity. This is whether – as in GA – the letter ‘r’ is pronounced in all words or whether – as in RP – it is only articulated when it is found before a voiced vowel sound.
Imitate an American accent
So, in RP, the R in ‘hard’ is silent, in GA it is heard. And similarly, the pronunciation of the word ‘gardener’ or ‘Dorchester’ in RP would contain no Rs , in GA, both would have two. In fact, when we imitate an American accent, that’s the feature that we’re most likely to change. Just like when we try and impersonate a Frenchman speaking English, we first tend to substitute an unvoiced –TH for /s/ and a voiced –TH for /z/ – “I sink so!” and “Zat’s right!”
Profound changes
When the first English settlers arrived in Jamestown in 1607, they would have sounded like their compatriots back in the Old Country. We don’t have recordings until nearly three centuries later, by which time some profound changes had taken place. What happened in the meantime and why?
Class and status
One thing we do know is that up to the American Revolution of 1776, we all spoke with a rhotic accent and there was probably little difference between English spoken in UK and the corresponding New World variety. Not long afterwards though, non-rhotic speech came into vogue as a signifier of class and status in England. Everyone wanted to learn this new fashionable accent, and because it was regionally unaligned, it quickly caught on around the Empire through institutions such as the armed forces, the civil service and then the BBC.
Plantation owners
In the US, former colonists, particularly in port areas such as Boston, which maintained close trading links with England, also adopted RP. From the Southeastern coast, RP spread throughout the South, following the wealth of the plantation owners.
Economic and political elites
After the Civil War and industrialisation, political and economic power passed from the ports and cotton fields to emergent manufacturing centres in the Mid Atlantic and Midwest. Pittsburgh, Cleveland, Chicago and Detroit were in their ascendency and English cultural and linguistic influence began to wane. These new hubs were populated by Scottish and Irish immigrants and other regions of the UK where rhotic English was still spoken widely. The new economic and political elites didn’t speak RP and, as a consequence, it lost its status. What resulted, General American, is now only spoken by a minority of Americans, mostly in the Midwest. Other areas have developed their own distinct variations over time
Thanks to Matt Soniak
https://getpocket.com/explore/item/when-did-americans-lose-their-british-accents