Her latest series, I Hate Suzie, has received favorable reviews from critics, so it would make more sense she’d want to devote time to a series she’s currently working on rather than re-treading old ground.
Another reason Billie Piper may not want to jump back into Doctor Who is that she’s been busy with various projects since she left.
Doctor Who will survive without her, though I’m sure no one would say no to a random one-off appearance in Season 13.