I wonder what made you come to this decision? Is it the demand of people wanting to do Level Leaderboards? (I doubt that is the answer though since these were separated Categories of each individual game, I'm just very curious though)

It was originally the super mario galaxy community that started it, because it made more sense to have all the runs together rather than split between two webpages. I don't remember ILs really coming into the decision, but that was an added bonus as those leaderboards just didn't exist before.