English has become a global language for a few different reasons. First, the British Empire visited, colonized, or began many of the populations over the world. Therefore, their language impacted more areas than any other language. English has also become a global language based upon necessity.
What is meant by this second reason is that the world is becoming more globalized. Businesses trade with other businesses all over the world. Scientists from all over the world meet to discuss important issues. People travel more. These reasons, while very limited, denote a reason for a global language to exist. It would seem that since the English language was already embedded in so many different ares that it would make sense to push for English to be the global language.
Outside of that, English has simply been readily accepted as a major global language. Schools all over the world are teaching it. Given that it has not undergone enough scrutiny, and has had no real opposition, people have accepted English as a global language.