In about 1490, the early printer William Caxton tells an interesting story. It seems that sailors whose ship had been becalmed in the Thames estuary went to shore to refresh their food supply. They happened on a farmhouse, and one of them asked the woman of the house if they could buy some "egges." The woman could hardly understand a word they were saying, and explained that she could speak no "frenshe." That upset the guy, who tried to say he could speak no French either. Then another in the party of sailors, who knew something of the dialect, broke in to say that they wanted some eyren. Sure, said the woman.
Both sides in this little drama spoke English. They just couldn't understand each other. One side talked about egges, the other about eyren. And, although the dialect gaps narrowed in England over the coming century, big differences remained. Those differences were the cause of the first regional accents in colonial America.
The first permanent settlement in Jamestown dates to 1607. Just 13 years later, the "Mayflower" came ashore in Cape Cod. They came from different places. The colonialists in the south came from England's "west country," the Puritans came from the east part of England near the North Sea. The western types typically pronounced an "r" that came after a vowel; those in the east didn't. Hence, New Englanders pronounced Harvard this way: "Haava'd."
Some echoes of those early accents remain along the east coast, but for the most part the United States managed to develop its own regional accents all on its own. For instance, I grew up in northwest Wisconsin, not too far east of the Twin Cities and the Mississippi River, and I have no trouble understanding that the worm in the backyard is an angleworm, a comforter is a quilt, and a boulevard is that grassy, city-owned strip between the sidewalk and the street.
On the other hand, I never heard anyone in the north talk about a chifforobe (a wardrobe), clabber (curdled milk), or goobers (peanuts). And I certainly never heard anyone utter, except in jest, y'all.