I think it's down to the innate conservatism of the production team. Whether it be RTD saying "No-one cares about planet Zog" and having people in the year 70 squillion still listening to Britney Spears and watching Big Brother, or Moffat asking "Why can't the Doctor date?" the whole attitude has been that people won't watch unless they have characters they can 'relate' to. Which means in turn that everyone must embody the unthought assumptions of early-21st century small-l-liberal (in the LC use of the word rather than real Liberalism) Western English-speaking people, whether those be positive (so the odious Mme du Pompadour had a black friend) or negative (so everyone is casually sexist whether in the future or the past).
It's a gigantic failure of imagination - a failure to see that the future and the past are both fundamentally different from now, and a failure to imagine that a different worldview is even possible (or that such a thing as a worldview even exists, rather than a bunch of inchoate prejudices).
No-one's interested in the strange and different, after all - the great RTD said so - and unfortunately not being unthinkingly sexist is strange and different at the moment.