There's no giant conspiracy to keep 2D versions unavailable to force people like you to watch it in 3D or wait... as if the theaters were doing well enough these days they could refuse on purpose to meet an existing demand!

The demand for 3D is just higher than you seem to believe, and the megaplex often picked their best rooms to make 3D-ready. You might be right more people in general either still dislike or don't care for 3D, but you'd be wrong to think more of the theater-going, blockbuster-loving people (the main customers of the theaters) will choose to see their movies in 2D over 3D. Most people just don't go to theaters anymore for their movie watching on any regular basis, and another percentage of people will only go when it's something special, or when they decide to go to a theater, they will choose to prefer to see the blockbuster in 3D and wait to watch the other releases showing at the time at home on BR/DVD/TV on demand and so on.

Many theaters get 2D copies right away, when they know they can't meet the demand for a release with only their 3D screens. They figure between the people who refuse to see 3D movies and the people who want to see the movie enough they won't turn back if they can't get in to see it in 3D, they can fill these too. Typically if you've noticed, it's always the 3D projections that are sold out first. It's tickets for those most people buy when they buy in advance.

Otherwise, megaplexes wait and pick up 2D copies when their 3D screening rooms no longer sell as well. They show a newer movie that will do better in their 3D rooms, and continue to show the other movie in 2D only. The turnaround is also faster, because it's more and more the same smaller percentage of people who go see movies at the theater on a regular basis.