I believe the historicals in the B&W days were associated with lower ratings and were dropped for that reason. Maybe that fear persists? I don't really know.
The "lower ratings" reason is an urban legend; historicals didn't do any worse than the more science-fictional stories around them. The historicals went away after "The Highlanders" (which I love) because producer Innes Lloyd didn't like the format and thought having monsters was best.
When I pitched a story to Declan May for Seasons of War 2 (which May cancelled), I pitched a pure historical because I thought it would be fun to write a pure historical War Doctor story. The pitch was rejected and, frankly, I felt like I dodged a bullet given the way things went with the Seasons project. The story, suffice it to say, involved a war, but I won't say which one. Maybe, some day, I can repurpose it for something else.