The major factors that lie behind this development are historical factors. They are the development of the British Empire and the later rise of the United States as the dominant economic and political force in the world after WWII.
The rise of the British Empire brought the English language to every inhabited continent. From North America (and places like Belize) to India to South Africa and Australia, people were speaking English as their native language (or at least as a language learned very early in life). This came about because of Britain's economic and military power.
When Britain declined and the US rose, English spread further. US imperialism brought the language to Hawaii and Guam and the Philippines. More importantly, US economic might made it so that people in other countries needed to learn the language as well. As economic globalization occured (another factor that is important), more and more people needed to be able to speak English in order to function in the world of international business. This was due to the dominance of the US as the major country to be relatively undamaged by the war.