It's simple. You're attacking your opponents hand which reduces what hits the board so the threat is less.
The problem with this is that your discard spells don't come online until T3 so your opponent has plenty of time to resolve threats that you're ill-equipped to answer. I understand the strategy, but you need more board interactivity. You've basically said that your aggro matchup is Anger or bust and that's not a good place to be in this metagame.
Monomania has to go; if your opponent has less than four cards in hand you're getting an overcosted Mind Rot or worse. I've played with the card and would estimate it's a dead draw in 19/20 cases. If it's number one on your list of cards to throw away with Obelisk then it should be number one on your list of cards to remove from the deck.
I don't think you need Graveborn Muse here, either. Your eight draw spells plus Charmbreakers should be fine to keep you gassed up and you really don't need the life loss.
The card you really want in here is Rune-Scarred Demon. This lets you run singletons of your powerful top-end cards because you can find them when you need them and he's a win-condition all on his own.
I also like megabeast's suggestion of Suffer the Past in a discard-themed deck so I would try to make room for it.
I would try this:
-4 Vapor Snag
-1 Banefire
-2 Graveborn Muse
-2 Monomania
-1 Obelisk of Alara
+3 Shock
+1 Suffer the Past
+4 Voyage's End
+2 Rune-Scarred Demon
You now have a powerful toolbox of Banefire, Suffer the Past, and Obelisk of Alara and you can get away with a singleton of each because of the Rune-Scarred Demons.
The Demons also search up Anger of the Gods or Inferno Titan against aggro opponents who manage to recover, and he increases your threat density because a 6/6 flier is huge.
The Banefire doubles as your fourth copy of Shock and you can be a bit liberal using it as creature removal because of the Charmbreaker Devils.
Thoughts?