It was a prequel to "The Daleks' Master Plan". At the time, Doctor Who was produced nearly all-year-round. The regular cast would get a week or two off during the season. Usually, it was only one cast member at a time(eg. The Doctor doesn't appear in 2 episodes of "The Keys of Marinus" However, with this episode, the whole regular cast got the week off. And it set up the suspense for DMP. In addition, one Season 2 serial("Planet of Giants") had been edited from 4 episodes down to 3, and they had to "catch up" one episode. It was better to do "Mission to the Unknown", than it was to stretch an existing script out.