It's not really viable for every single map that gets uploaded to be going up for ranked. Not every one charts/maps to reflect ranked criteria, especially some of the requirements are just kind of silly. Basically forcing a mapper to create a difficulty spread of easy/hard/insane or whatever is kind of meaningless since the star rating is useless to a certain extent. Also tying into that is the time constraint, I don't really know if it's changed now, but I remember the difficulty spread being forced if the map was shorter than 5m. All in all, I don't think you consider this useful for the overall game since it's mainly an issue for osu!mania players and it's also one of the many reasons people aren't willing to migrate over.