Why a language becomes a universal language has little to do with the number of people who speak it. It is much more to do with who those speakers are.

Latin became an international language throughout the Roman Empire but this was not because the Romans where more numerous than the people they subjugated, they were simply more powerful.

And later when roman military power declined, Latin remained for a millennium as the international language of education, thanks to a different sort of power, the ecclesiastical power of Roman Catholicism.

There’s the closest of links between language dominance and the political and military dominance; economic, technological and cultural power.
Without a strong power base of whatever kind, no language can make progress as an international medium of communication.

Language has no independent existence, living in some sort of mystical space apart from the people who speak it. Language exists only in the brains and mouths and ears and hands and eyes of its users.

When they succeed on the international stage, their language succeeds. When they fail, their language fails.
This point may seem obvious, but it needs to be made at the outset, because over the years many popular and misleading beliefs have grown up about why a language should become internationally successful.

It is quite common to hear people claim that a language is a paragon, on account of its perceived aesthetic qualities, clarity of expression, literary power, or religious standing.

Greek, Latin and French are among those which at various times have been lauded in such terms; so English is no exception.
English replaced French as the chief foreign language after Second World War.

It is often suggested, for example, that there must be something inherently beautiful or logical about the structure of English, in order to explain why it is now so widely used. It has less grammar than other languages, some have suggested.

English doesn’t have a lot of endings on it, nor do we have to remember the difference between masculine feminine and neuter gender, so it must be easier to learn.
Such arguments are misconceived; Latin was once a major international language, despite its many inflectional endings and gender differences.

French too has been such a language.
Ease of learning has nothing to do with it.
And as for the notion:  English has no grammar. A claim that is risible to anyone who has ever had to learn it as a foreign language.
A language does not become a universal language because of its intrinsic structural properties or because of the size of its vocabulary or because it was once associated with a greater culture or religion in the past, these are all factors that can motivate someone to learn a language, but none of them alone, or in combination can ensure a language world spread.

Indeed such factors cannot even guarantee survival as a living language as is clear from the case of Latin, learned today as a classical language by only a scholarly and religious few.

Inconvenient structural properties such as awkward spelling do not stop a language achieving international status either.

A language has traditionally become an international language for one chief reason, the power of its people especially their political and military power, the explanation is the same throughout the history.

Why did Greek become a language of international communication 2,000 years ago?
Why did Latin become known throughout Europe? 
Why did Arabic come to be spoken so widely across northern Africa and the Middle East?
The history of a global language can be traced through the successful expeditions of its soldier sailor speakers.

But international language dominance is not solely the result of military dominance.

It may take a militarily powerful nation to establish a language, but it takes an economically powerful one to maintain and expand it.

By the beginning of the 19th century, Britain had become the world’s leading industrial and trading country. By the end of the century the population of the USA approaching 100 million was larger than that of any country in Western Europe, and its economy was the most productive and the fastest growing in the world.

British political imperialism had sent English around the globe, during 19th century, so that it was the language “on which the sun never sets”.

During the twentieth century this world presence was maintained and promoted almost single-handedly through the economic supremacy of the new American super power.
Economics replaced politics as the chief driving force. And the language behind the US dollar was English.