Apart from the more obvious geopolitical reasons, It could be related to the slightly simplified or reduced nature of modern English. You may be interested in reading "Our Magnificent Bastard Tongue: The Untold History of English Paperback" by John McWhorter. It makes for an enjoyable read.
besides the obvious Imperial rule you might observe how the language of science has changed through history.
Specifically after WWII the international language has been English. It is relatively easy to learn as well so people with different backgrounds can communicate and exchange ideas much easier, although actual command of English may not be perfect.
The saying goes that the world language is not English but 'Broken English'.
Well, I don't think the English language is a unique language, but it is, of course, an important language in this day and age due to trade, politics and so forth. It is now surely the lingua franca across the world. My two cents only.