Почему язык так популярен? около 8 причин.

English is an international language.
English is the dominant language of business and diplomacy.
English is the language of study. The most famous Universities are in Britain.
Britain and America are highly developed countries, so they lead to spreading English (their official language).  
English is a language of the Internet and high tech.
English has an old history of its developing.
Shakespeare wrote his poems in English.