That is a good question SUDH. Some have said that it was to distance themselves linguistically from the old colonial power. I don't really know. Do you have any theories?
I thinks the Britishers followed one standard model shaped by the Royalty.
While the Americans were a melting pot with immigrants settling from continent and Africa and other parts of the world. They did not have inhibitions on accepting different usages of English which were influenced by mother tongues of different people.
Thus they arrived in a situation, where they had a different attitude towards usage of English and a mentality to accept such usage as common. And what is common usage in any language is considered 'stanadard' by the linguists.
Same thing is happening in India. Mumbai(erstwhile Bombay) has many immigrants settled from all states of India and they speak a brand of Hindi, which has Marathi overtone. Purity of language takes a back-seat and thus Mumbai has developed a brand of Hindi, which is punctuated by words from many languages and does not reflect correct usage of 'Hindi Vyakran'. The Hindi purists are furious over such usage, yet, it is Mumbai Hindi which has made Hindi more useful and popular across India and many cultures. It has its own unwritten grammar and can be understood by those who llive in Mumbai.
Same thing is happening in Delhi, where there is Punjabi overtone and mixture of words.
Last year I had been to Bangalore and common man speaks Hindi during travel and stay in Hotel(though it may sound surprising to people who think South India never speaks Hindi). They also have their own version of Hindi and it is influenced by Deccan(Hyderabadi Hindi/Urdu). I think the subject is going beyond the purpose of this thread, yet I am trying to find an analogy, how English developed during the years.