What I heard was that they didn't have enough time to get the 3D conversion done before the release date, so they're not releasing Part 1 in 3D at the same time as the 2D release.
Part 2 will still be released in 2D and 3D, and I'm pretty sure that Part 1 will be released in 3D a month or so before Part 2 is released. Mainly so they don't waste all the money they spent on converting as much of Part 1 as they have done so far.