Board Thread:Skyrim/@comment-11276487-20130727171132/@comment-9421524-20131122055247

You have to remember that the contracts that the Brotherhood were getting were through rumours of people doing the Black Sacrement, not the actual act of it, due to having no Listener. If someone does the Black Sacrement, it won't be answered until they hear rumours of it. They may not have heard about Aventus, and so the reason they didn't do the contract was because they didn't know about it.