I have a theory that English is popular because pronunciation encodes almost no information so it works well regardless of accent. Some asian languages, and even French, heavily depend on tone for understanding so are tougher for non-native speakers to communicate in. Butchered English can still be generally understood, thus it's position as lingua franca.
English is currently popular because money is always popular
If English wasn't as easy to learn as it is, it would have been destroyed though.
The absolute selling point of English is the fact that since it has no proper rules it's the "glue" of European languages, it's the bash of human linguistics.
Ugly, crude, nearly impossible to master if you're not using it daily and all it really does is pin together superior languages that actually have formal rules, but could never be as flexible as "common".
Yes, it enjoyed tremendous success due to the british empire, and continues to dominate thanks to the hollywood propaganda machine - and it owes about 90% of it's success to that. But it's important to note that last 10% is important too, and that is because English is an easy language to learn and it is able to evolve rapidly.