Because Beth decided that the DB should be "falling apart" without giving any info on it. Just like, for some reason, the are no Shadowscales left other than that one Argonian. I mean really, you just introduced this new, cool, order of Assassins to us, and now you just randomly kill them off without giving us any info as to why? wtf
There is information about it in one of the books, it tells the story on what happened to the Dark brotherhood and gives some pretty good clues on why it is in the current state. The first time I did the DB questline the story on what happened with them both in the past and as the quests was done was pretty clear. It isn't told directly to your face but there is enough pieces of information being given you to puzzle together the while thing.
For me Oblivion was the game where the DB really stood out, mainly to do with some really good quests. I do however also like Skyrim's version of it as it does do something different then Oblivion and I did enjoy puzzling the story together and understanding what was going on.