Answer:
There are many factors.
1. English is the most spoken language in the world:
English is understood and/or spoken by approximately 1000 million people. This means more than a quarter of the world speaking english.
2. International business, science, technology or research:
This areas are dominated by English.
One third of the world's books are printed in English. 75% of the scientific bibliography is in English.
Global corporate headquarters are predominantly located in financial centers in the United Kingdom and the United States.
An 80% of the usuaries in Internet communicate in English.
3. The Second World War:
After the Second World War the United States grew as a world benchmark expanding commercially to handle the business and supply that Europe could not offer because the war had left the continent in crisis.
4. Movies and Music:
Thanks to the growth of Hollywood most of the movies are in english. And most part of the music is in English too.
Punk and hippie movements are other causes that placed English in the first place.
5. It's easy to learn:
The vocabulary is easy to understand and its related with many other languages. It is a hybrid language.
