That's a horrible under-statement. The Smith-Morra seems sound (Karpov even said it was alright!), but with proper play black can sustain a fine position a pawn up. With this in mind, Sicilian players will generally learn a small amount of theory on this variation, thus taking away the surprise value (which alot of unsound gambits rely on, for example: Latvian, Blackmar-Diemer, Elephant, and the fascinating Halloween Gambit). Secondly, when I play a gambit, I want open files directed to my opponents king. White's open c-file won't help him at all with this, as black never castles queenside in the Sicilian. Since white's c-file, and black's c-file are both open, white won't be castling queenside either, so it can be harder to throw a pawn storm at your opponent's king. Yet white still gets good play, even with these things considered.