I think if you don't have a relationship with the parents, you naturally don't have a relationship with the minor children. I don't think it involves any going out of the way on anyone's part, it just happens as a natural consequence.
Alternatively, if part of the reason that the GPs aren't seeing their GC is because they are a negative force in their lives, then facilitating a relationship might seem undesirable.
My MIL played definite favourites with my children and that dynamic, including the way she scapegoated one of my children, was a good reason to not facilitate such a relationship. I would have been wrong of me to force those relationships. Of course, she would deny it if told this perspective.
The test is that my children are now mostly grown and my older children have not forgotten their grandmother and, as a result of their memories, want nothing to do with her. They are not shy about sharing their memories with the younger ones either. At this stage they are all making their own choices. Yet anyone who heard it from my MIL would think I was the evil, alienating DIL. Not that I ever made any decisions about any of this. It was just how it naturally unfolded and MIL cut us off, not the other way around. I guess I was really awful that she felt she had to do that. Or, that action says a lot in another direction.