It's probably a combination of things. People already knew and loved Bioshock 1, so more people bought it the first week it came out where a lot of people I knew waited a bit on Dante's Inferno because they weren't sure how good it was. Also there are some people out there that felt the first Bioshock was such a great game that it didn't need a sequel and didn't feel the need to buy it, for now at least. It's also getting knocked for being more of the same, which is kind of odd because 90% of video game sequels are basically more of the same. It's a good game, I think expectations might have been a bit too high though.