Maybe it has something to do with leaving one's home behind and seeing the way the rest of the world lives; those experiences, no doubt, inform one's writing.
Brit expat and author of The Alexandria Quartet, Lawrence Durrell, who left Britain for France, once said "Everyone loathes his own country and countrymen if he is any sort of artist." Well, I don't know about that - Stephen King certainly loves his Maine roots enough to feature the area in several novels - but travelling, at least, seems to do wonders for the creative soul.
Case in point: Check out this list of expatriate authors put together by The Telegraph. Just the sheer number of notable authors who have moved abroad is remarkable. (Keeping in mind, that, yes, one of those was a forced move due to exile.)
And, is it just me, or do a lot of writers move to France?