Outside of the US from what I have seen, most teachers of English teach British English. The reason for this from what I can tell, is simply that: British English is what they were taught, because that is what their teachers were taught.
English has become the official language of the world because computers are an American invention. Computer programing started and remains in American English the world over. The net started as an American owned (I don't know what to call it) idea? project? At the point in time that English became so important, the US was the dominant power in Business.
English learners generally learn the language to improve their life through such things as travel or better business/job qualifications.
In spite of what you may think, the difference is quite huge. I felt it is slighting an English learner to teach them only British English, specially if they are learning English for business. So I compiled only a couple of the Wikipedia articles on the subject-that is not a very extensive search-but it's already 250 pages that I gave to my teachers of English. So imagine how much material there really is, because of how large the difference really is.
There was a recent article that Webster's unabridged dictionary had just gone over 10,000 strictly American English words that originated in the United States. This does not include American English words from other languages. How many words do you think Webster's has?