I think the two main reasons are: 1: During the time of the worlds greatest economic expansion (1950 to present) the worlds biggest economy (USA) native language is English. If anyone wanted to do business with USA, English was required. 2: USA designed the basis of our international communications of today, primarily the computer and the internet, and so were designed to work with English. As other countries adopted standardised computer systems, they also chose designs based on the original USA designs. In time changes were added so that they could communicate with people of the same language, such as modified keyboards and different language entry systems, but the underlying system is still the same as developed during the 1960's in the USA. And so modern communications are more convenient when English is used.

If you see through History, it was United Kingdom that developped industries, technologies, literature, maths and the english language is everywhere in the world: Australia, India, USA, Canada, UK, South Africa... the english language it is the 3rd most spoken language in the world. The whole technology is being developped mainly by english speakers: USA! For example NASA (that builds spaceships) is a very famous american company! The biggest example of english power is that internet was invented by americans in the 2nd world war! That's why modern communication is in english: lots of speakers, brains in the world are english, huge companies are english and the english is easy to learn for majority of people! ;)

even though they are other international languages such as esperanto, or sign language or ... but english is widely used by people for international communications, I don't know the reason too like you :p