I think, at least from the smarter people, it's because of the sheer competition. Skyrim, MW3, etc. (EDIT: See Demi.)
Then there's tons of people who never played SR2 and consider the series just an inferior clone of GTA based on some time with the first game.
And then there's other people, like a few people I know personally who have seen the trailers and think it looks awful and "unserious" so they don't understand why anyone would even be interested in such a title.
Although I shouldn't count one person in that last group, he buys games like Bad Company 2 for the single player and refuses to ever touch the multiplayer.