Because he was awesome?
And the ratings still need to be taken in the context of terrestrial ratings in general, wherein I still believe Who does reasonably well comparatively speaking. Is it the ratings behemoth it once was? No, but let's be honest it hasn't been since the first half of Whittaker's first season, and that was a sharp upswing after several seasons of declining ratings under Capaldi.
Setting aside co-production/streaming partnerships I think the BBC has four options.
1. Let the show die. End it and make it clear it's never coming back. Unlikely.
2. Put it on hiatus, and not a year or two's hiatus, make it clear it'll be at least 5 years before it comes back. Possible, but you do have to worry about this being a cancellation by default. One can imagine a scenario whereby in ten years time vague messages about Who's return keep eking out but no one seems to be doing anything. Then it's twenty years....and so on and so forth.
3. Carry on as you are. You keep making the show but accept that it's never going to be as popular as it once was because there's way more competition now. You accept it is a middle ranked show that still attracts millions of viewers (just 3 instead of 7 million) and you tailor your expectations and your budget accordingly. Personally I think this is the way to go. Frankly if you offered me a season every year that was on the whole as entertaining as the last two seasons have been, I'd be happy with that, and you hope the show plays itself back into form/gets lucky with it's casting and finds itself with a new Ten/Donna Eleven/Amy/Rory dynamic and becomes super popular again.
4. The BBC double down. Who is a big show and needs to continue to be seen as a big show. You get a new streaming partner and you throw money at it. Get a name to play the Doctor, maybe get a new showrunner, new writers etc. It's an option for sure, but feels like a case of the gambler's fallacy/sunk cost fallacy (not quite sure which fallacy fits best). It may be that whoever writes the show and whoever is the Doctor, it just can't ever be quite as popular as it once was.
And yes I'm self aware enough to realise that my preference for option three is possibly me just putting my hands over my ears and going "na na na not listening' or being this meme in human form
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Except I think things are fine. Not brilliant, not amazing, but certainly fine. I still find the show enjoyable (for the most part). Do I wish RTD was a little less RTD at times sure, have I enjoyed Ncuti's run way more than Whittaker's, absolutely.