What changes accents is not ‘globalisation’, or, at least, not that alone. Nor is it as simple as TV or mass media. People (not all people, but most people) are influenced by the people around them. Also, accents have constantly been changing. We now know that the ‘upper class’ accent of Queen Elizabeth II’s daughter, Princess Anne, has shifted over the past 40 years towards a more ordinary ‘standard’ or ‘BBC’ accent.
The force that changes accent is probably the same that produces changes of species: natural selection. Over time, for example, the teutonic languages of the Saxon and Vikings gradually evolved into English, because of the contact between invaders and invaded. Before them in continental Europe the Latin language gradually shifted into French in one place, Italian in another, Spanish, Portuguese and Romanian in others.
Later, British colonial and criminal activity produced the accents of Australia, New Zealand and, by the way, the United States and Canada. It produced the accents of Republic of South Africa and Zimbabwe (the white populations, at least).
In the late middle ages English underwent a ‘great vowel shift’, the precise reason for which is not fully clear.
The mass media may have some influence. But it is often exaggerated. It is interesting that even though European continental rock, TV and film have been in the post war period dominated by American material, this did not produce an American style accent, as it has to a greater extent in East Asia. That is more likely to be a matter of how they are taught English and (in some cases) by whom.
This brings me onto the teaching of English. In the U.K. (or at least in England), there is one unique feature, mentioned by Greenbaum in his Oxford English Grammar: a non-regional upper class accent. This has been the accent of all upper (and often middle class people born anywhere in England and educated in private schools). This became standard English pronunciation.
So the published phonic rules that govern the teaching of literacy in compulsory schools are those of ‘standard English’ - often thought of as the ‘middle class accent’. That will tend to move many children in the Northern, Eastern and South Western parts of the country away from the regional accents of there parents. It is true that more and more schoolchildren do speak something nearer to ‘standard English.
This has not happened in Scotland, Wales or Northern Ireland to anything like that extent. This is partly because of national pride in the language. There are upper and lower class accents in Scotland, but they are all rhotic and manifestly Scottish.
I cannot speak for the US, but much the same may apply. The influences at work are a mixture of population mobility, mass media, compulsory mass education and whatever may be the prevailing approach to literacy (including phonics), fashion and, last but not least, the desire for social status.
The processes of natural selection are slow and imperceptible. But they can be accelerated by increase in agents of change: communication, education, population movement, and perhaps social attitudes and aspirations.
I shall probably be judged off topic for this, but there it is.