The vast majority of books on the English language take British English as their starting point. There are many reasons for this: British English is the old variety, and traditionally British English has enjoyed more prestige, at least in Europe and in former English colonies, than American English. Until not so long ago, American English was considered less educated, less cultured, less beautiful than British English. Teachers in many European countries were not allowed to have an American accent, and high-school students who returned from a year in the United States were sometimes punished with low grades by conservative teachers. This attitude seems mostly to have disappeared even if there are still traces of it.
Another reason for the predominance of British English is simply one of publishing traditions: Britain has a long tradition of producing textbooks and dictionaries and of marketing them all over the world, whereas there have been relatively few American textbooks or dictionaries written for audiences worldwide.
The result is a curious situation: The majority of the world's native speakers of English are Americans, about 240 million people. They make up the majority of the 400 million native speakers, compared with about 57 million speakers in Britain. They speak English with American pronunciation, vocabulary, and grammar. Much more American than British English is heard in films and on television, and more American English is used in international business, computing, and science. Yet the textbooks for non-native speakers that are used in schools in many countries are still mostly oriented toward British English, sometimes presenting American English as an aberration, often just in the form of a collection of words to learn. There are some useful books on American English, mostly designed for college or university use, that deal with vocabulary, spelling and pronunciation, but they usually give very little information concerning grammar or practical language use in the United States. Nor do they make any attempts to explain why there are differences between the two major varieties.
Summary
There're many reasons that most English-teaching books take British English as the standard, among which two reasons are more obvious. One reason is that British English has enjoyed more prestige, at least in Europe and in former English colonies. Another reason is that Britain has a long tradition of producing textbooks and dictionaries and of marketing them all over the world. A strange situation appears that nowadays most people and occasions use American English, while what is taught around the world is British English.