Because what it did with its atmosphere and story was more important that what it did as a shooter. I've played "better shooters" before, but none of them meant anything after I was done. Bioshock stuck with me because it accomplished something that I had never before experienced in a game.
To me, "shootin' dudes" wasn't the point at all. It's different for each person depending on why they played it and what they expected.
Currently playing:
Bloodbath Paddy Wagon Ultra 9