It's a pretty common theme in lots of other genres, too (I'd mention Star Wars, but it's really science fantasy anyway).
The author needs to convey a lot of information to the reader to introduce what is going on, keep some background mysterious, etc. It's much easier to do when your main character knows absolutely nothing about the surrounding world, because things are explained to the main character just as they are explained to the reader. If your main character is an expert in everything, it's hard to reveal things to the reader that are already resident in the character's head -- but if he's an idiot, and has to have things explained to him, it's easier to present things to the reader.
I just came back from a long drive during which we listend to two books that used the "farmboy syndrome", aka "main character is an idiot". One was Michael Crichton's State of Fear -- I kept wondering why this lawyer was getting dragged around on commando missions, before it clicked that Crichton needed his main character to be an idiot -- so he'd have an excuse to explain everything to the reader.
The other was a fantasy story -- Eragon -- that takes the farmboy syndrome literally. It's Star Wars meets Terry Brooks meets Ursula Le Guin meets David Eddings -- if there's a fantasy cliche, it's in there (I kept annoying my wife during the drive by popping up with Star Wars quotes appropriate to the story).