Because the majority of the player base doesn't want "ranked" playlists (look at any trends of ranked vs. unranked playlist population in not just Halo, but just about any game). This way, all the playlists can be "ranked" for those that want to look it up, and the majority that don't want that or don't care aren't bothered by it.
Personally, I'd prefer if the ranks were in the game, but I see why they are doing it the way they are.
I laughed.
This Halo 4 ad they keep showing in the MLG stream (and I've seen it several other places) would be nice if they threw a few lines of dialogue in there.