As I already stated before in another topic, they may have indeed reached the Great Valley; just because you don't see them in the ending doesn't mean they aren't there. And even if it was decided they wouldn't reach, keep in mind that this movie had more than on director; Don Bluth didn't make all the decisions and didn't have 100% control over the movie.
Now as for why we don't hear them being spoken of in the sequels, personally I think this decision was made to cut back on characters. From the first movie, other than Cera, Topsy's the only threehorn that gets a speaking role. Some of his personality is shown (he's racist, but still loving towards his daughters).
But Cera's mother and sisters were treated more like just extras. They're almost never seen and we don't have any clues on what their personalities or characteristics were like. So when a sequel was made, the new director (Don Bluth had nothing to do with the sequels) may have decided not to have the mother and other siblings in the movie because of this.
You could argue that Littlefoot's grandparents didn't speak much or appear much in the movie. But there is a key difference here. Littlefoot had been orphaned, his mother killed by a sharptooth. It would have been a strange move to have him suddenly live on his own despite reuniting with his grandparents. Universal had to give LF some kind of parental care, so that may be why his grandparents were brought over to the sequels.
As for the other parent characters, again I think this also applies. They can't have the characters be orphans, so they bring over the parents. Ducky's father might have been dropped (most of the time) so that focus doesn't get too stretched. When you have so many characters, it's hard to pay special attention to them individually. Ducky's father, as the movies suggest, is still there, but is absent most of the time due to directoral decisions most likely, to keep things simple.
If I were to offer an in movie explanation, it would be that maybe the males of Ducky's species don't stick with the family that long, and that most of the parental care is the mother's responsibility. The males might wander off, maybe patrol the area for threats or something, but if their mates or children really do need their help, they'll come back.
I can't think of much else to say at the moment.