Steven Moffat explains the reason why in his years of overseeing Doctor Who, the series has not had any female doctors.
I didn’t [cast a woman] because I wanted to cast Matt Smith and I wanted to cast Peter Capaldi.
While Moffat did not cast the first female Doctor in his tenure, it is cannot be denied that both Smith and Capaldi did a great job in their respective seasons.
But with calls for diversity in media intensifying over the years, campaigns for a female Doctor are now being pushed more relentlessly.