After King Alfred’s victory over the Vikings in 878, the government of Southern England came to be established in London, which later became the capital of the whole of Britain. Because of this, the English spoken in London and the East Midlands was gradually adopted as the ‘official’ variety of English. And as time went by, this dialect (and its later developments, profoundly influenced by Norman French), became the ‘standard’ language- the form of English generally accepted for use in government, the law, business, education and literature. Standard English, like all standard languages, is therefore largely the result of historical accident. If the Vikings, who held the north of England, had defeated Harold’s army, the capital of modern Britain might well be York, and this book would be written in (and about) a very different kind of English.
What is ‘standard English’?
Michael Swan. Practical English Usage (3rd Edition).