Well there’s no ads on the BBC, so ad revenue isn’t the driver here. There are on overseas markets, but my understanding is that foreign viewership is fairly niche.
Reading around a bit...
During his run, in the UK, Capaldi seemed to average a bit under 6 million viewers per episode. Early in Whitaker's run, she was pulling 7 to 8 million. For those same episodes, she got about 1.6 million viewers per episode in the US.
For comparison for US viewers, the new Quantum Leap is getting a bit over 2 million viewers per episode.