First, may I offer a few definitions before commenting on the phenomenon you ask about. A regional or geographic dialect is a variety of a language specific to a given geographical region. They evolve constantly and are identifiable by 3 qualities:
1. The Grammar used
2. The lexicon (or vocabulary)
3. The pronunciation.
In the view of most American linguists, all humans speak with a regional dialect. This reflects the linguistic habits of the place they live or grew up. British scholars are skittish about the word dialect. They prefer to use the word accent instead. British linguists use the word dialect only to describe forms of English that are markedly “non-standard”. Thus a British linguist will speak of a “Yorkshire dialect” only if the person is using Yorkshire dialect grammar and pronunciation and lexicon. If on the other hand, the speaker is “educated” and is using Standard English grammar, British linguists consider him to be speaking “Standard English” with a “Yorkshire Accent”. Which brings us to your question.
When we Americans think of British English, we are referring to RP (“Received Pronunciation”) You may have also heard of it as BBC English or “The Queen’s English”. When a regional dialect spreads among people of a particular race or class, regardless of where they live, a SOCIAL DIALECT evolves. In the UK, it is quite common for person raised speaking a regional dialect to learn RP as a social dialect to use in formal situations.
The British did not always use RP. It began in 1700AD. Before that, for example in 1600s, when Shakespeare produced his works and the King James Bible was made, there was no RP!!!! Scholars generally agree now thanks to vowel analsis in poetry that Shakespeare spoke in what we might think of today as a Mid Western American accent.
It is not understood where or why RP developed. Some speculate that it developed as a social mechanism among the English elite for distancing themselves from the colonists and other “riff raff” at the time. Writers at that time commented about the sudden insurgence of changes in vowels, but there was no “formal declaration” or anything like that.
Most Americans today speak in General American English (GAE). This is the English that most of the early settlers spoke. As populations moved toward the center or our continent, they carried with them the language of the time. However,
after RP developed newer “immigrants” brought with them the changes and often settled in the large metropolitan cities. Which is why we have “Boston accents” which resemble RP, but people in Minnesota still speak with what we would consider “the ealiest colonial sounding english”. The “Coastal South” regional dialect is a blend of GAE and RP. Shakespeare probably sounded not like Kenneth Brannaugh, but William H. Macy. It is hard to believe, isn’t it?!
Accents are still changing. The biggest regional changes in the US now is called the Great American vowel shift. The “aw” and “o” sounds are beginning to show some change. Ask your friends around the country if they pronounce these two words the same way or differently: Dawn and Don.
It is very important to remember that dialectical uniformity, not diversity, is the most striking feature of English pronunciation in North America. Our sound system is remarkably consistent. GAE can be heard in the most dialectally distinctive areas.