Cause the lingua franca always changes with the Superpowers of the world, English has been that lingua franca for decades possibly centuries if you count the UK's time in the limelight. I mean Chinese is seeing a boost in prominence in the last few decades, but at this point, English is super engrained for Western countries and specific spheres in Asia. As for why Japanese and German aren't as widely used, they are regional powers. The times of nationalistic and imperial empires are over for the most part they have become regional powers over time (in other areas it is global, but in terms of overall it's regional) because of the 20th century. Plus, there was a time when the lingua franca was split between English and Russian for a few years irc, it didn't last of course but it did happen.