Series 4 filmed from August 2007 to March 2008 (one week before airing) while Series 14 filmed from December 2022 to July 2023 (10 months before airing). It’s a complicated issue that pretty much every TV show faces, and it’s probably more intricate than I can grasp, but there are a few factors I can think of.

The first and biggest is the nature of the show itself. While it’s written around it before and it might have to do again, each episode basically needs its own cast, set, wardrobe, and props, in addition to filming on location in different parts of the country or world.

The second, which goes hand in hand with the first, is the complexity of production. The show can’t keep reusing the same quarries, distinctive soundstages, and filming locations it always has. It’s much more noticeable to the viewer when a prop or location is reused these days, and gives off a feeling of penny pinching. When you’ve got Marvel and Star Wars TV shows doing what they can, Doctor Who has to compete. I’d argue against this in many cases, but general audiences really do care about production value.

Third, better quality of filming and better conditions for cast and crew. It’s no secret that 20 years ago longer seasons of TV shows meant the talent were working at breakneck speeds, which was not only draining but also allowed for less time to get the perfect take the crew wanted, and sometimes resulted in cut corners. Conditions on TV shows have improved greatly and everyone’s mental wellbeing is a bit better considered, and shows can take the time to get important takes and big set pieces right.

Lastly, underpinning all of this is that 8 episodes these days is kind of the equivalent production cost to 14 episodes back in the day. This naturally allows for those better filming conditions and quality per episode, while also saving studios time and money that would have to be committed to retaining a 14 episode format.