There are, in my opinion, several factors that need to be
discussed.
First of all, the British Empire with the vast territories that it held, from
Europe to Asia, from America to Australia, from Africa to the oceanic islands
was a means for the English language to reach far and wide. Relationships had
to be established and maintained, people had to trade, disputes had to be
settled all over the British Empire and, as usual, the ones who held the power
dictated many of the terms.
Of course, the industrial developments and inventions that sprung from the
heart of the empire, England, contributed to the diffusion of English in
countries that were not part of the empire.
All of that continued with the birth and rise of the USA. They were never an
empire in the strictest meaning of the word, but they acted as if they were and
that helped English reach even further than it first did. Plus, with the rise
of the informational age and the appearance of what some consider to be the
greatest discovery of mankind since fire, that is, the Internet, things got a
new and even more stronger impulse.
Of course, what helped a lot during the last centuries was that scholars,
whether they were British or especially American, strode towards simplifying
the mentioned language and making it more user friendly. Moreover, English has
always had an openness towards accepting new words and incorporating them into every day usage.
