It's a simple problem: good writer does not mean good script writer.
I know one female novelist who would love to write for Doctor Who... but she's never written a script (four or more novels so far, but no produced scripts).
Suspect that if you really hunted for them, then there are a lot of female writers who could write very good Doctor Who.
Whereas Helen Rayner was, bluntly, slotted in to write the 2007 Dalek story when Moffat couldn't do it (he wrote Blink instead), and Russell T has said that he regrets that he didn't do more editing work on it (as he normally did, but he was too busy at the time).
But Helen Rayner writing a story (and doing it well, I thought, the next season) was basically an extension of the Casualty model of script editor-becomes-writer-and eventually becomes producer and finally executive producer (which is why Casualty and Holby City are now cursed with Oliver Kent as their ultimate boss). If the New York Dalek story was poor, that was more to do with the concept not working and needing a touch of genius to make it take off than any flaws in her script.