The answer's simple, he told Joystiq that he "felt like we said what we wanted to say about Rapture". He continued: "That's one of the reasons Bioshock 2 wasn't the right project for us and, as we agreed with the company, we didn't have the timeframe or the scale to make the product of the ambition we had."
But what did he think of the game which was developed by 2K Marin and released earlier in the year? "I think [2K Marin is] a very talented team," he told Eurogamer, "and I think it fulfilled the mission of completing the story of Rapture."
We really liked Bioshock 2, and we're beyond excited for Levine's Bioshock: Infinite. Bring on the Big Daddies.