Yeah, it happens all the time in TV. With only 43 minutes an episode, and only 22 or so a year, they can only spend a certain amount of that re-visiting old stories. Look at X-files or Buffy for example. Sometimes, too, the writers would love to bring someone back but the actor isn't available any more.
If we do get a season 6 this is one I definitely would like to see again, though.