I'm assuming the rolling production is the main reason for this, as people have said. I think another part is also that a lot of television shows allow cast and crew members to direct episodes so they can get a director credit, which allows them to pursue other directing work through the union. This is why you often see that star actors in shows have also directed the odd episode. I might be talking complete bollocks, but I think this is a thing, if anyone in the know can confirm?