The major factors that have caused English to become an international language are political and economic. English has become an international language because Great Britain, and later the United States, have been the most important powers in the world. Their power has caused their language to be more useful to people than any other language.
Both Great Britain and America have created empires, either formal or informal. The British domination of India, for example, made English a very important language on the Indian subcontinent. Later, the American domination of world trade and politics after WWII made it important that people in all corners of the world should be able to speak English.
In these ways, economics and politics have combined to make English the most international of languages.