Why can the English language be used so widely?

An opinion only:

England was at one time ruler of a large portion of the world. In these areas you had to speak some English to be able to have any dealings with the government. There was no allowance for mother tongues and indigenous languages in those days. This gave a large number of English speakers spread over the globe, in Australia, India, Canada, South Africa and parts of the far east, as well as in the USA.

The fact that the USA was English-speaking meant that when the mass media came into existence much broadcasting was in English. English speakers also dominated the development of mass transport. This is still a powerful motivation to learn English - pop music, Hollywood films, US TV series etc are all in English. Most pilots and ships captains use English, and much of the internet. There is no opposite pressure on English speakers to learn other languages.

Agreed

I'd just add that most of the countries where English is spoken as the native language are also propserous and engage heavily in internstational trade.

Being a native speaker of English can be very isolating.