English has become a world language because :
1) the British Empire made it an official or communication language in Canada, India, Pakistan, Bangladesh, Malaysia, Singapore, HK, most of Oceania, half of Africa and half of the Caribbean.
2) the US influence in the world since WWII
3) the richness of the English language, which by far the largest vocabulary of any languages in the world (estimated to be about 7x that of French or 4x that of German).
4) the flexibility of the English language. New words can be created all the time, nouns can serve as verbs and vice-versa ("do you yahoo ?", "I google this word....")
5) English's strategic position in Europe as a half-Latin, half-Germanic language, which makes it easy to learn most Western Europeans.
6) English's flexible pronuciation. It may be a drawback as there are so many different accents that it makes it more difficult to learn or understand, but it allows for more diversity, personalization, and tolerance over "mistakes" from learners (unlike French which should sound just as a native to be accepted by French people).
7) Easy, no-nonsensical grammar (almost no genders and plural, little conjugation, fexibility of the sentence structure...)
8) Recogniton of English as "official" language of computing, sciences, technologies, aviation, etc.
9) Success of music in English (rock, pop, rap, etc.) - not just from the UK and US, but also from countries where English is not an official language (Sweden, Denmark, Japan, etc.)