Don't know, possibly because Rogue trashes cards from your opponent and Graverobber trashes your own cards.
I guess it adds insult to injury to play 2 Rogues with an empty trash and gain that juicy Grand Market from your opponent right to the top of your deck. If you play 2 Graverobbers this way (trash first, gain with the other), you can just get your own GM back.
Edit: Ninja'd.