They likely don't see it as enough of an Oscar contender to risk losing box office in a very crowded Christmas calendar, including The Hobbit, Zero Dark Thirty, This Is Forty, Les Miserables and Django Unchained. Being a 3D release, perhaps The Hobbit is the biggest concern as the two films would be competing directly for the same screens in a lot of cinemas. And seeing as both will be distributed by Warner Bros. there would have to be some kind of nice balance found, and perhaps that balance has turned out to be unworkable.
But by positioning Gatsby in the Summer, as opposed to January or February say, the studio seem to be showing confidence in the film's ability to draw a crowd. I'm not surprised – Luhrmann's certainly got the razzle dazzle down and this is a pretty marketable cast; Leonardo Di Caprio, Tobey Maguire, Carey Mulligan.
There's no exact Summer release date announced as yet, but whenever it is, it already feels too far away. Well, you know what they say: all bright, brash melodramatic things come to those who wait.