I once sat through a lecture on this (and broader considerations of the differences between "British" and "American" English) by some world-renowned linguist whose name I can't for the life of me remember. Here's a potted (or potty) version of what he had to say. His thesis was also backed up by the TV programme "The History of English" many years later, again presented by a linguist whose name escapes me.
No arguments about spelling. Noah Webster had his way. Also, most of the modern "americanisms" such as aluminum/alunimium are usage issues.
You have to go back to Elizabethan times to get a handle on what happened in the USA. First of all, almost everyone with education in England in the sixteenth century would have had an accent similar to what we consider today to be the Devon brogue, the "Arrr, but the arrrnswer do loie in the soil" type of thing. A matter of degree, the linguists don't appear to agree how pronounced this burr was. It was further influenced by the Cornish and ?Devonish sailors who apparently made up the majority of the crews for ships which plied the Atlantic, too. RP is a relatively recent invention.
When the early settlers went to America they took their accent with them to Jamestown and other areas of Virginia. The hypothesis is that the accent spread from there. Being in relative isolation for a long period of time (about a century), the accent didn't change all that much. By the time of the major wave of immigration it was well established as the "way to speak", and the incomers adopted it rather than corrupted it (although some corruption probably did occur - look at the range of accents across the US today).
The theory goes that the most pure form of "original" accent is endemic in inland Virginia and on the islands off the Carolina coast, where the outside influences were negligible for many years.
Effectively, then, the American/Canadian accent is closer to "pure" - whatever that is - than English in England or anywhere else. Historically speaking, of course.
For what it's worth ...
