Why not any earlier? Well, Lucas is wanting to make sure he'll be able to capitalize on the 3D releases to the fullest extent. Right now, while there are quite a few 3D theaters located throughout the United States and all over the world, it is only likely that the number will grow over the next years. Not only that, but 3D televisions will possibly make a big splash with consumers as the prices come down and as the technology itself improves.
This Star Wars nerd is pretty excited. Not having the opportunity to EVER see any of the original Star Wars films in the theater has always haunted me. When the Special Editions hit the theaters in 1997, I was in junior high school and my mother and father would not take me to the theater to see the films. This is a chance at redemption...I only have to wait another half a decade!