Oh, I was prepared to defend its storytelling and the brilliant use of interactivity in the plot twist, but you're talking in comparison to the sequel?
Put simply, it's the atmosphere of Rapture. In the first one, we had never seen it before. It confused us, it terrified us, and it riveted us. It was compelling and original, and exploring it was exhilarating. Now, on subsequent playthroughs, you're reliving those wonderful experiences.
In the second one, the experiences were new, but the place was not. The wonder you first experienced while exploring Rapture for the first time is gone, replaced with a familiarity of your surroundings and a full understanding of what Rapture is and what it means. The story was compelling enough, but the setting had lost all its originality. That's the main reason.