Here's a theory I came up with: during Mufasa's reign, leopards lived in the Pride Lands until Scar took over. When Simba took his rightful place as king, the leopards moved to the Back Lands instead of the Pride Lands.
I had that same thought after reading about that on the leopards article recently. It would certainly make sense for Scar and the hyenas to drive out and/or kill any other big predators in favor of being able to glut themselves on prey they would otherwise have to share. Assuming they started soon after he ascended to the throne, it would make sense that by the time Kion was around there wouldn't have been any left in the Pride Lands.