I found this interesting discussion between Paul McGann and one of the Big finish team (not good at recognising voices and he doesn't introduce himself) in the bonus track to "An Earthly Child", the new Big Finish play with him as the Eighth Doctor and Carole Ann Ford reprising Susan. (download extract here):
McGann: And I didn't know either that it's an old character, or at least that the idea that the Doctor had a family goes right back to the beginning, doesn't it, right back to the early 60s, during Bill Hartnell's reign. Interviewer: It does. I mean, the very first episode of Doctor Who was William Hartnell as the Doctor with his granddaughter Susan, played by Carole Ann Ford, and that - McGann: Of course - Carole Ann who's in this now! Interviewer: Yes! McGann gasps. Interviewer: She's the same character. McGann: Now that, I didn't realise. Interviewer: Yeah! McGann: So there's all this, sort of, symmetry... Interviewer: Yeah! McGann: God, how fantastic! It's the next generation of where she left off. Interviewer: Yeah! It picks up from exactly the same place where she left the series in 1965. McGann: That's... just too spooky. But kind of great as well, isn't it!? Interviewer: Marc [Platt]'s done a pretty seamless job in carrying on the story from where it finished. McGann: But tell me why, em - let me interview you for a minute - why did we... I mean, I've played Doctor Who, why didn't I know that Doctor Who had ever had kids, that there was a family involved? Is it something that people are coy about, is it something that's not spoken of, is it a bit of a taboo subject? Interviewer: I don't know, actually. I suppose it's a bit of an area of the show that some producers have chosen not to go in. McGann laughs. Interviewer: I have to say Russell T Davies and the team at Cardiff were very supportive of us doing this story, I mean they were very helpful. So it is - McGann: Initially I felt, well, is it because somehow the Doctor over the years has become a kind of ascetic figure, you know, there's something... we can't imagine the Doctor having, we can't imagine the Doctor having sex, or something. There's something el- you know what I'm saying, so, is that the reason? I'm just trying to figure out why, why don't people talk about this kind of thing? Interviewer: I suppose what it could be is, if the Doctor has a family, it domesticizes him... McGann: Perhaps. Interviewer: ...in a way, so it gives him roots and ties him down, whereas he's always portrayed as this wanderer, and quite an isolated figure at times. McGann: Right. Interviewer: So... McGann: It's me that's sex-obsessed, then. Interviewer laughs. McGann: But that's a good point, then isn't it - that it perhaps somehow goes against the grain, and also it kind of rings true, you can see, you know, even, presumably as Bill Hartnell's character developed and then later on into the other actors that played him, it was more impotant that the Doctor, as you say, remains a kind of wanderer, you know, someone who - Interviewer: A drifter. McGann: A drifter, yeah. Somebody essentially quite rootless.
Funny that McGann, though obviously very aware of Hartnell's Doctor, had never before heard the Susan parts of the back story. But I think he and the other guy successfully identify why the Doctor has never since been portrayed as having a family.