As Seth said, maybe he did not want to put Disney secondary characters. But somehow, perhaps in the reign of Scar arose more lionesses with their cubs. For example Mheetu, right? I know that my answer is not correct but I have helped

Well, I read in a lion king book: "Simba ran down to his mother and also saw that his friend Nala and her mother Sarafina were visiting from another pride" but there still could be two because in the the lion king at the start as you see cub simba running through the cave to his parents, you see a small cub sleeping. It's definately not Nala because the cub has coloured toes, so we don't find out who it is.
Hope this helps.

I guess Disney did not want to introduce too many side characters, so they only focused on main chars. And they also did not want movie to be too long. Beside they did not really show much of Simbas free time. In most of the stuff he did, he probably would not want other cubs near, like escape to Elephant Graveyard, or other cubs would not belong in scene, like in that tour around kingdom he did with his father. Inserting them to existing scenes also might draw too much attention of what should be in focus and redirect it on background. So I guess they did not see it as necessary to add more cubs.

But TLK Six New Adventures books do introduce some more characters which are suppose to grow up with Simba and Nala. Also comics introduced few more. So there are ways to find at some cubs with whom Simba and Nala were growing up with.

I guess same reasons go to Simbas Pride. Also you must understand that Simbas Pride was direct to DVD sequel, and they probably found it better for budget to do less cubs. Also again they did not show much of Kiaras free time as cub, beside those necessary for story.