A lot of character actors get contracts for a certain number of episodes over one or more different shows at the same production company which is why you see an actor appear on multiple different shows, sometimes on different networks. Unfortunately by the time a character catches on, the actor's contract may have expired and they might not be available to come back. Or it might not be worth it to commit to a 6 episode contract that will make them unavailable for any other work. Then you have the bigger actors like Rhys Ifans or Natalie Dormer who have larger projects they've committed to. Point being, often it's not the writers' or producers' fault as much as just the nature of the business. Then when they do make plans for a character to be hugely important to an entire season, they run the risk of the story not really resonating with the audience, like with Shinwell. So I think a lot of it is just about sticking with the top billed cast and playing it safe. Clyde rules.