Sometimes, the Ninth Doctor’s series doesn’t seem so long ago, but that aired in 2005. Twelve years! That’s a long time in TV years.
That’s a lot of time for character development, exploring themes, and even calling back to previous stories and characters (villains, too!).
I can’t speak for other fans, but I feel like Doctor Who has exhausted a lot of content in the last 12 years.
That’s not a slight to the writers, by the way. The show has become comfortable. Not necessarily a bad thing, but it could be better.
Introducing a female Doctor changes that. It’s a fresh take on what we already know; a new lens to see the universe through.
What’s the TARDIS like with a female Doctor at the console? What’s the Doctor’s dynamic with her companions?
Instantly, we have new things to explore, but we also have the usual things to get used to with a new Doctor. Personality quirks. Sense of humor. Being open or reserved.
Doctor Who has always been about change. It doesn’t matter who the Doctor is — it’s still the Doctor.