Actually, Americans still have the original British accent. We kept it over time and Britain didn’t. What we currently coin as a British accent developed in England during the 19th century among the upper class as a symbol of status. Historians often claim that Shakespeare sounds better in an American accent.
No. Not much above is actually true. Let me try to explain in the most simplistic way how language change happens when one language gets exported to a far-away politically-separated land. (This is not meant to be a strict lesson in history, although there are some parallels)
Let’s say we have a group of people who live on Circle Island that speak dialect A of Circlish. They live uncontroversially under the reign of King Circulo III.
But after some time, King Circulo starts getting a big head, and acting tyrannical, and a group of persecuted Circlians escape, setting sail for Square Island, the land of freedom and easily-purchased fire-arms.
These renegade Circlians settle down, and establish the debatably ineffective Democratic Republic of Square Island. They are no longer Circlian Pawns of the King, but liberated Squarites led by George Pentagonian. However, they still speak the same dialect A of their separated Circlian brethren.
Over the centuries, the two far-removed peoples continue on their separate paths, interacting every now and then, but for the most part minding their own business. The speech of the Circlians changes little by little, as languages always have. Certain sounds become dropped in speech. Others are added. The popular slang of one year is replaced the next year by the hippest words this side of the Triangle River. Before you know it, a distinct dialect B has emerged, replacing the now obsolete dialect A.
Meanwhile, the same thing has been happening to the speech of the good polygon-fearing citizens of Square Island. But their dialect has changed in different ways. Where some sounds have been dropped over in Circle Island, they have been retained on the other side of the Oval Sea. Different expressions fall out of fashion, and some things said on Square Island, in what can now be labeled as dialect C sound downright strange to the subjects of reigning monarch, Her Circulosity Queen Cerclea IV. And vice versa.
Of course students on both sides of the pond are still required to read literature from good ol’ King Circulo’s time, and they recognize most of what is said, but other things are less clear, and some passages understandably confuse the most intelligent speakers of dialect B and C. Critically no one speaks the “original Circlish.” In fact, it is hard to pin down exactly what would be the original language—because it had been changing since before writing was invented all those years ago in the small hamlet of Elliptico.
However, all these good folks can still communicate with each other, which is very helpful for Circle-citizens who watch Squarish movies (which dominate the market), and for Square-citizens who watch Circlish sci-fi shows involving evil salt shaker aliens that are intent on eliminating the mysterious Professor What.
More centuries pass, and the two dialects of this language have both continued to change. They’ve changed SO MUCH that now the people of these two Isles can no longer talk to each other in what used to be their native dialect. Only the most well-studied professors of dead languages can still read the old tongue of King Circulo. In Circle Island, phrase books are sold to potential tourists to Square Island, so they can talk to the local Squarites in their own language, Squarish. On Square Island, they continue watching Circlish television and cinema, but only with subtitles.
The two peoples of these islands now speak different languages, that are not mutually intelligible.
So there you have it.
Now back to the real world.
Neither Americans nor the British speak with the “original British accent”, whatever the hell that is. While the accents in Great Britain did indeed ‘develop’ in the 19th century (and the 18th, and the 20th, and the 15th-17th… etc.), so did the accents in North America after colonization. They changed in both places, and WITHIN both places (The Deep South, New England, Scotland, Wales, South England, etc all have distinct accents/dialects).With regards to the great Bard, no one speaks his dialect of English today, but if you listen to clips of what his pronunciation may sounded like, honestly, it sounds a lot like Scottish English (to my ears at least), which still doesn’t mean that they or anyone else has the “original British accent.”
Good day! :)