When Peter Capaldi revealed on Monday that he would be leaving Doctor Who after the next series, it came as a surprise to many fans. "I've never worked the same job for three years, and I feel like now is the right time to move on,” he told BBC Radio 2.

Naturally, the announcement has sparked a wave of speculation; it has been variously suggested that the BBC were desperate for him to stay on, and that they pushed him out. But Capaldi's portayal of the character has been met with widespread acclaim from the critics – so why is he quitting a job he so clearly loved? Here are some possible reason:

The show's ratings were falling

During Capaldi's time on the show, its overnight ratings have fallen considerably: only 6.1 million people tuned in to watch the live broadcast of the last Christmas special, while the David Tennant-era episodes were regularly drawing more than 10 million live viewers.

The rise in online catch-up viewing is partly responsible, and this is undoubtedly the case with a show like Doctor Who, that skews towards a younger demographic; consolidated viewing figures for the show tend to be much higher. Accounting for catch-up viewing, the audience for 2016's special was a slight improvement (0.1 m) on Capaldi's previous Christmas outing. But there's another number that matters just as much to the BBC: the Audience Appreciation Index, or AI, a score out of 100 which measures viewers' enjoyment. If a cult sci-fi show is drawing a smaller audience, but that audience adores it more than ever, the public service broadcaster can justifiably call it a success. Sadly, this isn't the case for Doctor Who.