Well, English is a pretty simple language, grammatically. Compared to its Germanic neighbours, it doesn't have gendered nouns, it doesn't have the funky word order of German, nor the funky word order of the V2 Scandinavian languages. It has fewer letters in its alphabet, and it has fewer phonemes.
It does have a very large vocabulary, mostly due to the French influence, and it's in dire need of a spelling reform to fix all the accumulated crap, but those are hurdles for writing English or mastering English. When it comes to speaking simple English, the bar is lower than what is required for speaking simple German, or simple Norwegian, for example.
It's also pretty forgiving when it comes to word order, which means understanding bad English is also easier. So I think there's some sort of lowest common denominator merit to it that made it easier for it to take over.