This is why I get so annoyed when threads come up where people want to keep a father's id from their child. The children miss out on opportunities to form relationships.
I'm not going to judge anyone, because there can be good reasons for keeping fathers out of the picture, and every case will be different.
In any case, 40+ years ago, things were different - there was still a lot of shame associated with having a child out of wedlock, and abortion was still illegal, so to take that option, you had to break the law and risk your life, and many young women weren't given a choice.
My mother apparently had a massive falling out with her parents about the baby, but apparently she really went for the whole hippy "turn on, tune in, drop out" thing, with heroin and everything, so I've no idea whether her getting pregnant was just one more thing to add to the list, or what. In any case, I think she carried massive resentment about it all through the rest of her life, and never really dealt with it. She was clean by the time she met my father, but she had problems with alcohol at various points, and I think that was partly so she didn't have to face up to how she felt about these things. I think when we were small, though, she was just getting on with her new, settled life and putting the past behind her, and trying not to let that shape the way she now was. I don't know, because as far as I'm aware, in the 11 years between me finding out and her death, I don't think she was aware that I knew about it.
However, on a purely practical level, when would have been a good time to bring it up? If you haven't been mentioning it from day 1, so you grow up always knowing about having a half-sibling, when do you tell? Clearly we (my younger full sister and I) needed to be old enough to have some understanding that we could have another sister who didn't live with us, but it's something which would probably cause loads of questions and knock-on effects, because you don't know how people will react - and once you've told others, you can't control them not telling their friends, their parents' friends, other family members, possibly people you don't want knowing, because you remember the shame and difficulty. And what if your children react badly when you tell them? You don't want that to coincide with starting school, changing school, having to deal with that alongside one of them having to have hospital treatment, your own hospital treatment, starting secondary school, approaching GCSEs, A-Levels, university, settling into first jobs - there's always going to be something for one or both of them, and over the years, it might not even be something you think about that often. We didn't have a very great relationship when I was a teenager, and telling me something like that then, I don't know how I'd have taken it, but probably a lot worse than 10 years later when I did find out, and at a more critical time in my life, educationally speaking. It was bad enough at 26.
So if you don't tell children from the outset, then I can see how it's never really going to be the right time. It has all made me think honesty and openness is the best way forward, even when it's about bad stuff, but I can also see how it's never quite the right time, and if you're not good at handling emotional stuff in the first place, it's easier to put it off ... again.
(Not that I am completely forgiving of my mother, and I have had a lot of counselling/therapy to try and straighten some things out in my brain, but equally, some of it is just down to being human.)