I think the main reason is that this is the first time it's been possible (due to the ease of long distance travel and communications) to actually have a global language. The dominant linguae francae at other periods of history (Koine Greek in the Hellenistic period, Latin in Roman empire and the Western Middle Ages, French up to c. 1900) weren't global languages, so much as dominant regional ones (and all of these examples are from Europe and the Mediterranean, since that's the area I know, but I'm sure Arabic, Mandarin, Swahili etc. have played similar roles across the world) because there was no need for the entire world to speak a dominant language: they weren't connected enough to need one.