I dont think they could get a good story together or something. I know Will Smith and Spielberg were involved but they pulled out and killed the project...
Around the world, a collective sigh of relief is breathed by otaku everywhere. Steven Spielberg and Will Smith had been conspiring to remake the Korean film Oldboy (which was based on a Japanese manga). The absurdity of the idea seemed to encourage Spielberg more, saying he was actually not remaking the film, but adapting the manga. Ignoring the fact that they were casting Will freaking Smith in it, I think we all can say this was one Hollywood adaption we were really unsure about.
Last we heard, the film was being put on hold as Futabasha and Show East argued over the rights. Now, Slashfilm is reporting (via the Latino Review), that the film is dead. Dropped quickly and unceremoniously into the coffin, the film was kicked after Dreamworks and Mandate (the Korean production studio) couldn't agree on the adaption. Dreamworks and Spielberg simply walked away from the deal and left it be -- apparently it was more trouble than it was worth.
Given that even Steven Spielberg couldn't get the film up and running, I would guess that we're not going to see this being revived by any mad scientists anytime soon.