It just seemed to fit. I don't have the RPG book, so don't know officially where he's from - but it's a mining planet, which fits with "damp lung", sounds kinda like a miner's problem (I grew up in a mining area), it's remote enough for the accent, plus there's the whole what happened to the place in the movie thing. Jayne seems very comfortable with the folk there in the movie - it's as good a place as any to be home.