This is a fascinating question. I think there's a common answer and an uncommon answer.
The common answer starts with the industrial revolution, which itself started in England, making that nation into the largest empire the world had ever seen (controlling about 25% of earth's land mass) and bequething their language as well as scientific, academic and legal influences around the world. Of course the U.S. (which could be argued is also an empire) expanded in all these areas, especually culturally (through movies, music, etc.) and economically.
But here's my uncommon answer: English is an incredibly dynamic language, an amalgam of linguistic influences that go back over a millennium.
Think about the English lexicon. 80-85% of words come from greek and latin. (Showing the influence of those empires). Some of the basic subject and personal pronouns (like they and there) and other words (like know and knife and sky) are viking. And of course, there are influences from French as well, although the grammar is germanic to its core.
So, while English may be the lingua franca right now, it's worth while to remember that the English language is a living pathwork of linguistic influences.