Their reasons I suppose, like any of us do who are estranged from family. As the children grown up, they’ll realise that family dynamics can be complicated, like we all do. If the children continue to not have contact with them, they’ll be strangers to them. They’ll have their own lives with their parents, friends, education, work, hobbies etc. They’re unlikely to sit around spending all their time thinking about people who are strangers to them, they’ll just get on with life.
My children are older, they know the reasons why we’re estranged from some family and they love and trust us enough that we made the right decisions about that.