Why do MOST fantasy worlds have a "common tongue" that EVERYONE knows how to speak, and why are they so popular?

It seems to me that 90% of the time there is a universal language that every character can understand, and even if they don't it seems like they can learn it in little to no time at all. I can't really think of any instance where a language barrier is actually in play in a fantasy story, the only instance I can think of is in LotR, where it takes Frodo YEARS to learn elvish.