Moss plays the slot. Gaffney isn't ideal there. I think its probably as simple as that.
Gaffney is also TERRIBLE in terms of YAC, which Shanahan values. With RG3, Moss should tear it up in terms of YAC. He led the league a year or two ago in that category I believe.
---------- Post added April-18th-2012 at 12:38 PM ----------
Edit: Unless you're just responding to the guy talking about a custom Griffin #10 jersey. Sorry!