Well, technically they were just "postponed," which everyone took to be a nice way of saying "cancelled," but we don't really know that.
I bet Disney is calculating the best move here.
On the one hand, I could see a 3D trailer for a re-release really blowing away modern audiences if done correctly. It would be like the modern equivalent of the x-wing flying out of the tv screen in that first SE trailer from late '96.
On the other hand, there's the potential for backlash leading up to Episode 7. Rolling out the SE yet again, even if it's a more polished "final" version, might do more harm than good.