User blog comment:PlantyThePottedPlant/Random Topic Blogging III: Blog Harder/@comment-1659929-20130513233826/@comment-1518158-20130513235339

I think it was more of a heritage thing. When Mufasa's parents were still around, either Mufasa and Scar would take his place after he died. Mufasa took his place, so if anything happened to Mufasa, Scar would take over for him. But once Simba was born, Mufasa had an heir, so they no longer needed to resort to Scar, the would-be king, to take over for him since he has a more direct replacement, which is why Scar tried to kill Simba and Mufasa in TLK. Had nothing to do with Mufasa/Sarabi's ability to reproduce, me thinks.