Why does mccrum says that we should not talk of english but any englishes

McCrum says that "we should not talk of English but of many Englishes" because the English language usage spans across multiple nations. We have the British English, American English, Australian English and New Zealand English. We also have business English and simple English. There are also different English usages and lexicon wherein a Native English speaker won't have heard of.

Thus, when you plan on migrating to a country, you ought to learn what specific kind of English language they speak.

we should not to talk english but many english