Voice acting is a very expensive undertaking. Would it be nice to have each in-game region have a distinct accent? Sure - but it'd cost an immense amount of money, especially when you consider that they're already paying to have voice acting for many different languages.

The English voice acting improved considerably as of Heavensward and beyond. As a British guy, it's very nice to see the various different accents associated with the UK reflected in such a prominent game.