I think everyone gave great explanations to these questions.  Spanish interestingly enough is also widely spoken in the Western Hemisphere and is slowly encroaching into the US as a second language, especially California, where I live. I don't like the language myself (I don't know why. It's just sounds ugly to me but I've got nothing Spanish-speaking people). Spanish got here from early Spanish colonialism. My favorite second language is German, but it's riddled with English words, unfortunately. I don't know why. Not not ist IT words of the last 35 years. Maybe Germans just love English or like showing off their English skills.  I've always wondered about that.

I frankly wish English weren't the lingua Franca because all it does is discourage native English speakers from learning foreign languages and makes them lazy about it. And in so doing, it disconnects them from other cultures and the richness of other languages. The best thing I've ever done for myself is learn German and break away from that English mode. 

Your question almost sounds like a joke, because "lingua franca" literally refers to an actual language used in the Middle Ages which was a derivative of Italian.

But, yes, I agree with Saqib and Paul. It's because the British Empire was so powerful and dominant in the 1800s and early 1900s, and because the United States succeeded it as a superpower. I think it is not only that it was recent, but also that it was the language of machinery and technology and manufactured products.

It was common to say that "the sun never sets on the British Empire" meaning that British possessions were so widely scattered around the globe that it was always daytime somewhere in the British Empire. 

Researchers have come up with an interesting concept that explains why the most "important" languages are not necessarily the same as those with the largest number of native speakers:

http://tinyurl.com/ze4n2ux

It has to do with the idea of "hub" languages, one that work for communicating between pairs of other languages.