I think Disney might have started to doubt 3D earlier, but now they see that 3D is something as popular as they where expecting it to come more & more as more time went by.
That would explain why come Disney cancel the 3D re-release of the Little Mermaid, just to uncancel it recently, and still premiere during this year after all. And why The Lone Ranger is not in 3D.
Its a shame that "The Lone Ranger" is not in 3D because I have seen the 2D trailers of it, and it would have been even better in 3D like The Great Gatsby, but even greater need & use of 3D.