It's acclimation. Since the 70s, the BBC has produced series for a particular number of episodes and then dropped them. There are some exceptions, like Doctor W

It's acclimation. Since the 70s, the BBC has produced series for a particular number of episodes and then dropped them. There are some exceptions, like Doctor Who, but that has been the practice. In the US, historically, we just keep running until the ratings tell us to quit.